Americans have pretty much always felt entitled to make law for themselves. As Virginia royal governor Alexander Spotswood complained 60 years before the Declaration of Independence, "by their professions and actions they [the colonials] seem to allow no jurisdiction, civil or ecclesiastical, but what is established by laws of their own making." That position was vindicated by the Revolution and remained unchallenged in any serious way for two centuries. Today, however, there is an advanced and determined movement afoot that-through the mechanisms of international law and super-national institutions-does challenge the right of the United States to define its own legal obligations as an independent and sovereign nation-state.
The Founding Generation, of course, knew international law and recognized its importance in facilitating relations between states. They readily accepted that, as an independent sovereign, the United States was bound by international law to the same extent as were the other "powers of the earth." This much was made clear by the Declaration of Independence itself, which explained why it had become "necessary for one people to dissolve the political bands which have connected them with another and to assume among the powers of the earth, the separate and equal station to which the Laws of Nature and of Nature's God entitle them."
In 1776, international law was considered, philosophically at least, to be a species of Natural Law: the "law of nations." Such law could be discovered or discerned in the practice of states, but it could not be "made" in the manner of domestic or municipal legislation. Then, as now, there was no global body politic and no global legislature. Consequently, as was necessarily implied by the inherent equality of every independent state, no state or league of states had the right to establish the legal obligations of any other state. All were equally competent to determine and interpret international law for themselves. As a result, and in no small part because international law did not purport to govern any state's internal affairs, American democracy flourished in this world despite being virtually alone in its republican institutions.
Global politics have, of course, been transformed many times since the United States declared its independence. In the post-World War II era, and especially since the Cold War ended, a widening swath of world opinion has come to view international law and institutions as inherently superior to national ones, as the very font of legal and political legitimacy, and as a proper and appropriate means of achieving change even within national borders. The following quotation, from a German Foreign Ministry description of the newly established International Criminal Court (ICC), perfectly captures these attitudes:
It is a monumental achievement in the field of international legal policy that individuals who have transgressed their obligations to the international community as a whole may be held responsible by an independent international judicial institution. The ICC thus symbolizes jurisdiction exercised on behalf of the community of nations.
At the same time, it is also fair to say that, beyond a few academics and activists, most Americans do not look to international institutions or the "international community" for validation of their government's actions or their own. One might well ask, in response to the German Foreign Ministry, what is the "international community"? Does it, for example, include China's Communist rulers or the Persian Gulf's divine right monarchs? And what obligations, exactly, might Americans have to them? Law, in the United States, is made by our elected representatives, and the measure of its legitimacy is the United States Constitution.
As a result, of course, international law has never been treated as a rigid and imperative code of conduct by U.S. policymakers. This attitude toward international law transcends political ideology and party label. Nowhere was it better displayed than in an exchange between then Secretary of State Madeleine Albright and her British counterpart, Foreign Secretary Robin Cook, during the run-up to NATO's 1999 intervention in Kosovo. As reported by Mrs. Albright's spokesman James Rubin, when Cook explained that British lawyers objected to the use of military force against Serbia without U.N. approval, she replied simply "get new lawyers."
Mrs. Albright's suggestion was perhaps undiplomatic, but it revealed a firm grasp of the essential genius of international law: It is a body of norms made by states for states, and its content and application are almost always open to honest dispute. Moreover, and most important of all, there is no global power or authority with the ultimate right to establish the meaning of international law for all. Every independent state has the legal right-and the obligation-to consider and interpret international law for itself. In other words, when questions are asked about the meaning and requirements of international law, the answers will probably, and properly, depend on who the lawyers are.
This does not mean that international law is illusory or that it can or should be ignored by states in the day-to-day exercise of power. It does mean, however, that international law is best viewed as a collection of behavioral norms-some arising from custom and some from express agreement, some more well-established and some less so-that it is in the interest of states to honor. As Chief Justice John Marshall explained in 1812 in describing one important aspect of international law:
The world being composed of distinct sovereignties, possessing equal rights and equal independence, whose mutual benefit is promoted by intercourse with each other, and by an interchange of those good offices which humanity dictates and its wants require, all sovereigns have consented [to certain legal norms].
The key, of course, is consent. Ultimately, the binding nature of international law is a matter of the consent of sovereign states. They can interpret that law in accordance with their understanding and interests, they can attempt to change it, and they can choose to ignore it-so long as they are prepared to accept the very real political, economic, and even military consequences that may result. This is the essence of sovereignty, which itself is the basis and guarantor of self-government.
This paper is designed as a short guide to international law for American policymakers. The topic area is, of course, vast-even when the inquiry is limited to what is commonly known as "public international law" (the rules governing the conduct of states) rather than international trade relations. As a result, the scope of the material treated here is necessarily limited and selective. An effort has, however, been made to discuss the most important tenets of international law as it is today applicable to the United States and to identify the current controversies over this law's interpretation and application that most profoundly divide the United States from its European Allies. In fact, the understanding of how the world's nations are, or should be, organized in their inter-relations and what role international law and judicial institutions should play in that great endeavor is one area where differences between the United States and Europe are growing rapidly and are likely to produce increasing future tension and diplomatic conflicts.
Definition of Terms
I. What Is International Law?
Perhaps the most important and vexing question about international law is whether or not it is "law" at all.
Traditionally, international law existed as a collection of principles and practices-some based on custom and some based on treaties-that govern the interactions of sovereign states. As a theoretical matter, most commentators found the basis of this "law of nations" in some form of Natural Law. As noted by Emmerich de Vattel in the 18th century, "We must then apply to nations the rules of the law of nature, in order to discover what are their obligations, and what are their laws; consequently, the law of nations is originally no more than the law of nature applied to nations."
Whether the actual practitioners of statecraft ever took the "divine" or "natural" foundation of international law very seriously, at least after the emergence of the "Westphalian" state system in 1648, is debatable. Over time, most states have complied with these rules in accordance with their needs and interests, always keeping in mind that violations of accepted norms can carry significant consequences-up to and including war. However, from the perspective of current debates about the nature and role of international law as an organizing principle, the most important characteristic of the traditional international legal system is that there was no regular means of judicial enforcement. All sovereign states are equal in law, and none can claim the right to adjudicate-in a definitive legal, as opposed to political, sense-the actions of another.
Changing this state of affairs has been one of the most important goals of "progressives" and "internationalists" since before the First World War. In particular, throughout the 20th century-and especially after World War II-determined and sustained efforts were made to establish some form of international judicial system under which states would no longer be the ultimate arbiters of their own international legal obligations. These efforts, which can fairly be said to include the League of Nations (and its Permanent Court of International Justice), the United Nations' International Court of Justice (ICJ), and the International Criminal Court (ICC), have always found favor with the United States at their inception but have always been rejected in the end. (The United States, of course, never joined the League, withdrew from the ICJ's compulsory jurisdiction in 1986, and "de-signed" the ICC treaty in 2003.)
The reason is simple enough. A genuine system of international law, comparable to domestic legal systems in its reach and authority, would require a universally accepted institution entitled both to adjudicate the conduct of states and, by extension, their individual officials and citizens and to implement its judgments through compulsory process with or without consent of the states concerned. Such a universal authority, however, would be fundamentally at odds with the founding principles of the American Republic. It would require the American people to accept that there is, in fact, a legal power that has legitimate authority over them but is not accountable to them for its actions.
Pending this revolution in American beliefs and principles, U.S. officials and diplomats should recall two basic points in their approach to international law:
As an independent sovereign, the United States is fully entitled to interpret international law for itself. The views of international organizations, including the United Nations, other states, and non-governmental organizations (NGOs) may be informative, but they are not legally binding unless, and only to the extent that, the United States agrees to be bound.
Any institution or individual invoking international law as the measure of U.S. policy choices is only expounding an opinion of what international law is or should be. That opinion may be well or poorly informed, but it is not and cannot be authoritative. There is no supreme international judicial body with the inherent right to interpret international law for states.
In short, the United States, like all other states, is bound by international law; but, like all other states, it is also entitled to interpret international law for itself. Whether the U.S. or any other state has been reasonable in its interpretation is ultimately a political determination.
II. Does the U.S. Constitution Acknowledge International Law?
Advocates of various international norms, real or imagined, are quick to assert that international law is part of American law and therefore binding on the United States government. This is true as far as it goes. There are, however, numerous caveats that must be taken into account in determining the extent to which international law considerations may, or must, inform American policymaking.
At the outset, it is worth noting that this rule is a judge-made doctrine that does not actually appear in the Constitution's text. The Constitution does, of course, make treaties "the supreme Law of the Land," although not as a means of empowering the courts to oversee the formulation and execution of United States foreign policy. The entire text of the Supremacy Clause makes its purpose clear-the targets were the states and not the federal government:
This Constitution, and the Laws of the United States which shall be made in Pursuance thereof; and all Treaties made, or which shall be made, under the Authority of the United States, shall be the supreme Law of the Land; and the Judges in every State shall be bound thereby, any Thing in the Constitution or Laws of any State to the contrary notwithstanding.
As Justice Joseph Story noted in his 1833 exposition of the Constitution:
It is notorious, that treaty stipulations (especially those of the treaty of peace of 1783) were grossly disregarded by the states under the confederation. They were deemed by the states, not as laws, but like requisitions, of mere moral obligation, and dependent upon the good will of the states for their execution.
The Supremacy Clause was designed to ensure that the United States spoke with one voice on the international level and that the states could not choose for themselves which federal treaties to honor and which to ignore.
Supreme law notwithstanding, however, treaties remain subject to the Constitution and to later federal action. Where there is a conflict between the Constitution and a treaty, the Constitution prevails. Moreover, treaties can be applied directly by the courts only to the extent that they are "self-executing" (most are not) or have been the subject of implementing legislation. Finally, Congress can modify or eliminate a treaty's effect, at least as a matter of domestic law, by a later statute. American courts are bound to respect the plain meaning of such a law even if treaty partners claim that this would violate U.S. international obligations and the claim is accurate. In this regard, however, it should again be emphasized that such a claim may or may not be correct in any given case, since no other state, group of states, or international institution is entitled-absent specific U.S. consent-to interpret or adjudicate American international law obligations. A difference of opinion over the meaning of either a treaty or the requirements of custom does not automatically amount to a violation of international law by any of the parties involved.
In addition, treaties are subject to a number of presidential actions. The President is the "sole organ" of the United States in its external relations. Although a President can "make" a treaty only after obtaining the Senate's consent (by a two-thirds vote), he can terminate a treaty (in accordance with its terms), or abrogate the agreement entirely, on his own authority. Similarly, the President can-as a lesser power-suspend American performance under a particular agreement as one means of achieving U.S. policy goals. Of course, all of these actions may be more or less controversial, depending on the circumstances.
In fact, arguments have occasionally been advanced that the President must obtain the consent of Congress-or at least the Senate-before fundamentally changing U.S. treaty obligations. However, these claims have not been successful, either with the executive branch or before the courts. The leading case is Goldwater v. Carter, where a group of Senators and members of the House of Representatives sued to prevent President Jimmy Carter's termination of the Mutual Defense Treaty of 1954 between the United States and the Republic of China (Taiwan). The United States Court of Appeals for the District of Columbia Circuit ruled that the President, as "the constitutional representative of the United States with respect to external affairs," was within his constitutional authority to terminate this treaty. For its part, the Supreme Court never reached the merits of this question. It vacated the D.C. Circuit's opinion and ordered the original complaint dismissed-an act strongly suggesting that this and similar questions are not subject to judicial determination at all.
Finally, although international law is generally considered to be part of American law, the United States, like other sovereign nations, can derogate from the accepted rules. And, like other aspects of the nation's foreign relations, the exercise of this authority falls-at least in the first instance-to the President. The Supreme Court's ruling in The Paquete Habana is not to the contrary, although claims are sometimes made that it is. That case involved the U.S. Navy's capture, during the Spanish-American War, of fishing boats in Cuba's coastal waters. The Supreme Court was called upon to determine whether these vessels were lawful captures and concluded that they were not. Citing generally accepted rules of international law suggesting that coastal fishermen were not to be molested by belligerent forces, the Court ruled that the boats were not lawful "prizes" of war. However, in doing so, it specifically noted that "where there is no treaty and no controlling executive or legislative act or judicial decision, resort must be had to the customs and usages of civilized nations." The suggestion is clear that, had there been a formal decision by the President (or by Congress through appropriate legislation) to ignore the otherwise applicable international rule, the United States courts would have been bound by that decision.
III. How Is International Law "Made?"
International law is made by and through the actions of states. This is true both with respect to customary international law and, since a treaty's meaning and continued efficacy greatly depend upon how the parties interpret and apply its provisions in actual practice, with respect to conventual or treaty law. However, for the sake of clarity, these fundamental aspects of international law will be addressed separately.
Customary International Law. Customary international law grows out of more or less consistent state practice over time. There is no hard and fast rule on how general a practice must be to be considered customary or on how long it must be followed. However, the "failure of a significant number of important states to adopt a practice can prevent a principle from becoming general customary law though it might became 'particular customary law' for the participating states." Moreover, a rule cannot be imposed on a state that has objected.
In this connection, it also is important to note that what are sometimes called the "sources" of international law are, in fact, merely evidence of what the law may be. This includes such authorities as (1) the decisions of international courts and arbitral bodies, (2) the decisions of national courts ruling on international law questions, (3) the writings of international law commentators, and (4) the statements of governments. As the Supreme Court cautioned long ago with respect to the writings of jurists and commentators, "Such works are resorted to by judicial tribunals, not for the speculations of their authors concerning what the law ought to be, but for trustworthy evidence of what the law really is."
Opinio Juris. Opinio juris is a critical element in transforming an international usage or practice into a binding norm of customary international law. Unfortunately, opinio juris can be as elusive as the Philosopher's Stone. The full term is opinio juris et necessitatis, and it refers to a belief by states that the practice at issue is legally required. In other words, however longstanding and widespread a practice may be, it is binding only if states comply out of a sense of legal obligation. As explained by Ian Brownlie, "The sense of legal obligation, as opposed to motives of courtesy, fairness, or morality," is "a necessary ingredient" in turning general usage into a legal requirement.
Derogation from International Law Rules. States can derogate from customary international law rules and from treaty obligations. Such derogations are considered to be different from a repudiation of the rule or treaty and must also be distinguished from differences of opinion over the actual requirements of international law or the proper interpretation of a treaty. A genuine derogation involves one or more states acknowledging the force and effect of a particular rule or provision but nevertheless departing from it in limited circumstances. As such, openly admitted derogation is relatively rare. Most often, derogations involve states agreeing (expressly or by implication) to depart from a general rule in their own dealings with one another. These states generally are not considered to have violated international law.
A state can also choose to derogate from an otherwise applicable requirement on its own account. Depending on the rule in issue, however, it will risk prompting a negative response from its treaty partners or from the community of nations at large. Whether such a state can be said to have violated international law by its derogation, however, is almost always debatable. This is a function of the manner in which international law is made-based on the actual practice of states. Determining whether a particular state has violated its international obligations or has merely set out to promote and establish a new and different rule (or treaty interpretation) that, in its view, may be superior requires augurs of exceptional ability. As a result, and as a practical matter, the question is very much a political one-ultimately resolved by whether or not other states follow the new rule.
Jus Cogens. There are, of course, certain rules of international law from which, it is said, no derogation is permissible. These are generally referred to as "jus cogens" or "peremptory norms of international law." The application of either term to a particular rule or practice should sound alarm bells for any American diplomat, since the benefits of achieving jus cogens status for a preferred rule are substantial. In fact, the number of international norms that can honestly be characterized as jus cogens-based on long and consistent state practice-is small. Thus, the impermissibility of the oceanic slave trade is jus cogens not merely because it has been universally condemned, but also because the responsible maritime nations have, at least since the mid-19th century, acted seriously and effectively to suppress the activity under a generally acknowledged claim of right.
Moreover, like other aspects of international law, jus cogens is subject to the development of new norms. As one important commentator has explained, "They are rules of customary law which cannot be set aside by treaty or acquiescence but only by the formulation of a subsequent customary rule of contrary effect." In short, the doctrine of jus cogens is subject to being formed and reformed by the actual practice of states. As a result, a principle that is claimed to be jus cogens but is widely ignored is probably not a peremptory norm of international law-however important the policy it may support or detestable the practice it purports to forbid.
Treaties and Other International Agreements. On the international level, any agreement between or among states can properly be described as a treaty. These instruments can be bilateral or multilateral and create binding legal obligations for the states that become parties to any particular agreement. Under international law, states are required to comply with their treaty obligations. The principle pacta sunt servanda ("keep your agreements") is often identified as jus cogens, and with some justice. All things being equal, over time, states have recognized the importance of compliance with their treaty obligations, and-in the absence of special circumstances-most at least attempt to do so. The unilateral abrogation of a treaty without sufficient legal cause is considered to be a violation of international law. Most recent treaties, however, contain a termination or withdrawal clause permitting a party to end its obligations by meeting a notice requirement.
Bilateral treaties are, of course, agreements between two states generally governing aspects of their relationship to one another. The interpretation and application of such treaties is a matter for the parties alone, although the agreement may well provide for a type of arbitration or adjudication in an international body-such as the ICJ-in case of dispute.
Multilateral treaties involve an agreement between more than two states, and these types of agreements have significantly increased in number and importance over the past century. They include such basic instruments as the United Nations Charter, the North Atlantic Treaty, and the Geneva Conventions, as well as a whole array of critical agreements governing all aspects of transnational commerce and relations. Examples of such agreements include the Vienna Convention on Consular Relations, the Convention for the Unification of Certain Rules Relating to International Transportation by Air (the "Warsaw Convention"), the agreements establishing the World Trade Organization, and the Berne Conventions for the Protection of Literary and Artistic Works.
Multilateral treaties usually establish a specific number of ratifications necessary before the agreement will go into effect among the parties (the Rome Statute of the International Criminal Court, for example, required 60 countries to ratify before it went into effect) and are often-although not always-open to accession by states that may wish to become parties at a later time. Like more recent bilateral treaties, multilateral treaties often provide for a formal mechanism-submission to the ICJ-for resolution of disagreements over the treaty's interpretation. States may or may not accept these provisions upon ratification. It is important to note, however, that there is no general principle of international law suggesting that an interpretation favored by a significant number of state parties to an agreement, even if this involves a substantial majority or near unanimity, must be accepted by all parties.
Treaties Purporting to Codify International Law. An increasingly important "source" of international law is treaties that purport to "codify" customary international law. These instruments must be treated with extreme caution, since they are very often much less than they appear. The codification of international custom is, in any case, a speculative business. States are far more likely to agree on general principles than on detailed provisions. Moreover, and more to the point, states are often much more willing to state a rule as internationally binding than they are to apply it in practice.
Nevertheless, in certain areas, serious attempts have been made to reach agreement not merely on principles, but on the details. Prime examples here are the Vienna Convention on the Law of Treaties, the Law of the Sea Treaty, and the 1977 Protocol I Additional to the Geneva Conventions of August 12, 1949. All of these agreements indisputably include some provisions that are, or can legitimately be argued to be, customary international law. Significantly, however, the United States has not ratified any of these agreements, and it is not bound by them-except to the extent that their provisions restate binding customary norms.
In assessing the effect of these and similar documents on the United States, it is critical to keep in mind that the mere fact that some provisions of a treaty restate binding norms of customary international law does not mean that the entire document enjoys that status. Each provision must be judged independently to determine whether there is sufficient state practice (that is, actual observance based on a sense of legal obligation and in relevant circumstances) to justify its identification as binding custom. Thus, although Geneva Protocol I Additional clearly restates certain customary rules, such as the rule against deliberately targeting civilians, it also includes many provisions that represent efforts to "move" the international law of armed conflict in a particular direction-specifically toward "privileging" guerrilla or irregular combatants. The United States rejected this treaty on that very account and cannot now be held to these provisions merely because other portions of Protocol I are binding custom.
Executive Agreements. Although all agreements between or among states can accurately be labeled "treaties" for international purposes, this is not the case with respect to American constitutional law. The President can make treaties for the United States only with the Senate's consent. However, he can also enter certain "executive agreements," which bind the United States internationally and also have the force and effect of law on the domestic level. The full extent of the President's authority in this area is unclear, although executive agreements have generally been "of a routine character."
Pre-ratification Obligations: Article 18 of the Vienna Convention on the Law of Treaties. One of the more vexing issues arises because of the practice, engaged in by both Democrat and Republican Presidents, of signing international agreements that have little or no chance of approval by the Senate and therefore will never be ratified by the United States. There are many reasons for this practice-it may appear prudent at the time to exercise "leadership" on a particular issue, or it may be an effort to drive international law in the direction an Administration favors. Regrettably, this practice often leads to claims that the United States is bound by a treaty that it has not ratified, at least to the extent that it cannot take action to defeat the treaty's "object and purpose."
This rule is drawn from Article 18 of the Vienna Convention on the Law of Treaties, which the United States has signed but has not ratified. Although it is often stated that the Vienna Convention "is largely a restatement of customary rules," emphasis must be placed in the word "largely." Article 18 is, in fact, a rule characteristic of civil law legal systems. Whether it can be applied to common law countries without express consent is debatable. Moreover, its application by American courts would raise significant constitutional issues, at least in any instance where the President's own authority was insufficient to bind the United States to a particular obligation, since treaty obligations cannot be undertaken without the Senate's consent.
In any case, in construing Article 18, it is important to note that the obligation it imposes is emphatically not to comply with the terms of a treaty before the instrument is ratified. Rather, it requires only that a signatory "refrain from acts which would defeat the object and purpose of a treaty"-suggesting that only actions deliberately calculated to undermine a state's ability eventually to comply, including and especially any uniquely irreversible action, are forbidden. Nevertheless, the potential application of Article 18 must always be considered and is one very good reason why any responsible President should not sign agreements he does not expect to be able to ratify.
IV. How Is International Law Interpreted and Enforced?
As states are the ultimate authors of international law, they also are the arbiters of its meaning. As suggested above, each nation, as an independent sovereign, has an equal right to interpret international law in general and its own international legal obligations in particular. The interpretation of one state-or group of states-is no better or worse than the interpretation of others. This does not, of course, mean that states can interpret international norms to a point where any actual obligation is illusory. They must act, especially in construing their treaty obligations, in good faith. Moreover, all states must understand and accept that their interpretation of international legal requirements may carry consequences. As a legal matter, however, there is no state, group of states, international organization, or judicial authority with the paramount right-paraphrasing Chief Justice John Marshall's description of the federal judiciary's power in Marbury v. Madison-to say what the law is. There is no international Supreme Court.
International Judicial Institutions. That said, there are numerous international judicial institutions that, depending on the circumstances, may well be entitled to issue binding judgments against states. The most important of these, of course, is the ICJ. The authority of these courts, however, is based on the consent of the states concerned-consent that can be withdrawn in appropriate circumstances. Thus, for example, the United States withdrew from the ICJ's "compulsory" jurisdiction in 1986. As a result, it is subject to the ICJ's rulings only to the extent that some independent treaty provision vests that court with the power to adjudicate a dispute between the United States and one of its treaty partners.
In addition, with the exception of the ICC and other, ad hoc, international criminal tribunals (which can issue orders directed at individuals), international courts have no direct means of enforcing their judgments. As a general rule, they must depend on the voluntary compliance of the relevant states or seek the assistance of appropriate political institutions. The extent to which duly entered international judgments (where jurisdiction was appropriate) may bind the courts of the United States remains an open question-even though the issue was before the Supreme Court, in the case of Medellin v. Dretke, in 2005.
This case involved the Vienna Convention on Consular Relations, a treaty to which the United States is a party. Among other things, this treaty requires that foreign nationals be permitted certain access to their country's consular authorities in case of arrest in the territory of another state party. A number of Mexican citizens have been convicted of capital crimes in the United States without having been granted this access-largely because it was unclear to local authorities either that the individuals were foreign citizens or that they wished the assistance of Mexican authorities. In any case, the Vienna Consular Convention does vest the ICJ with the authority to resolve disputes between parties, and pursuant to this provision, Mexico successfully sued the United States in that court. The ICJ issued its decision in 2004, determining that the United States had violated the treaty and ordering it to provide some means of reviewing and reconsidering the convictions of the effected individuals.
The Supreme Court accepted certiorari to determine the extent to which this decision actually bound the federal and state courts and whether the ICJ's interpretation of the Vienna Consular Convention should, in any case, be given effect as a matter of judicial comity. In the meantime, however, President George W. Bush issued a memorandum indicating that the United States would comply with the ICJ's order by having the state courts "'give effect to the [ICJ] decision in accordance with general principles of comity in cases filed by the 51 Mexican nationals addressed in that decision.'" In light of this determination by the President, the Supreme Court dismissed the case without deciding whether U.S. courts must implement properly entered ICJ decisions.
International Political Institutions. Although states are entitled to interpret their own international obligations, all members of the United Nations have agreed to abide by certain decisions of the United Nations Security Council-at least when that body acts in accordance with its power under Chapter VII of the U.N. Charter. Chapter VII vests the Security Council with the authority to "determine the existence of any threat to the peace" and to "decide what measures shall be taken." These measures can include diplomatic or economic sanctions, up to and including the use of force. U.N. member states are required to "join in affording mutual assistance in carrying out the measures decided upon by the Security Council."
Of course, the Security Council is a political, not a judicial, body, and it is far from clear whether- even exercising its Chapter VII authority-it can articulate or establish a member state's legal obligations. As a practical matter, however, the Security Council's political decisions may well be sufficient to impose a particular result on one or more states regardless of the legal principles at issue-assuming that all of the Council's permanent, veto-wielding members determine to act with a sufficient level of force. Moreover, U.N. member states do have a legal obligation to comply with properly entered Security Council Chapter VII resolutions as a matter of treaty.
Other Means of Enforcing International Law. In addition to international judicial and political institutions-both relatively recent innovations- the more traditional methods of enforcing international norms include diplomacy and force. It is clearly the case that, over time, most disputes over the meaning and application of international law have been resolved through diplomatic means. This is preferable to other means, since it generally preserves the dignity and sovereignty of the relevant parties. Force, of course, has always been the ultimate sanction, as it remains today. In the past, states have often considered a violation of international legal obligations to be a casus belli, and state practice suggests that this remains true today- even in light of the U.N. Charter's admonition that disputes be settled by peaceful means. (Although practice over the past 50 years would also suggest that, apart from actions taken in self-defense, states are expected to seek U.N. assistance in resolving disputes before resorting to armed force on their own account. At a minimum, this certainly appears to be the Charter's fair import.)