Top Iranian officials just threatened to inflict "harm and pain" on the United States, vowing to "use any means" to "resist any pressure and threat" over its nuclear program. It's not just rhetoric. Iran is making preparations to deliver on both promises by expanding its alliance with its evil twin, Syria.

With each passing day, Syria's Baathists and Iran's radicals inch further into each other's embrace, limiting our policy options - and making both less susceptible to international pressure.

The rising Damascus-Tehran axis means more trouble for the U.S./Israel in the Middle East, more Iranian/Syrian support for terrorism and insurgency across the region - and, worst of all, the specter of nuclear cooperation between the two.

Strategic ties date back to the early 1980s, when Syria lined up with Iran against Iraq (i.e., Iran-Iraq war) and the United States and Israel (i.e., Lebanon). Relations have grown increasingly chummy more recently - especially since Mahmoud Ahmadinejad became Iran's president last September.

Visiting Damascus in mid-January, Ahmadinejad emphasized improving relations - noting that he and Syrian President Bashar al Assad had adopted identical positions on all international issues.

On that same trip, Ahamdinejad also paid a return visit on the terrorist "tools" who had paid homage to him in December in Tehran, including Hamas, Hezbollah, Islamic Jihad and other Palestinian rejectionist groups, promising them they could count on Iran's full support.

Tehran-Damascus shuttle diplomacy is more frequent, too. Iranian Vice President Parviz Davoudi visited Syria just a couple of weeks ago for a meeting of the Joint Syrian-Iranian Higher Committee - the regimes' consultation/coordination organ.

Davoudi called for both sides to exchange views regularly, characterizing current relations as "strategic." After meeting with Davoudi, Assad crowed that bilateral ties were both broad and promising - and called for "all-out relations."

The Iranian and Syrian economies are increasingly integrated, too - boosting both powers' ability to buck any new punitive sanctions. They recently inked a series of trade/financial accords, including developing a joint banking system and building an Iranian oil/gas pipeline across Iraq to Syria.

On the security side, Iran and Syria concluded a mutual defense treaty in 2004, meaning they will protect each other if attacked. Reaffirming the pact last February, Syrian Prime Minister Muhammad Naji al Otari noted, "Syria and Iran face several challenges, and it is necessary to build a common front."

Mohammed Reza Aref, then Iran's vice president, responded, "We are ready to help Syria on all grounds to confront threats," undoubtedly referring to the pressure on Damascus following the killing of former Lebanese Prime Minister Rafik Hariri.

As the Iranian nuclear crisis came to a rolling boil earlier this year, Iran and Syria further cemented their security relationship with more meetings, undoubtedly discussing defense strategy should the United States take military action against either or both.

Both countries actively support terrorist/insurgent groups in Iraq, Lebanon, Afghanistan and the Palestinian territories, too. Their growing intimacy will improve their sponsorship's efficiency/effectiveness, especially after Hamas' recent electoral victory.

But the most dreaded strategic aspect of their partnership is the potential for nuclear cooperation. Last September, Ahmadinejad announced a willingness to share "peaceful" nuclear technology with other "Islamic" states. (Damascus is the most likely recipient of Tehran's nuclear largesse.) There is reason to be concerned. While Syria only has a small nuclear R&D program, based on a Chinese-supplied 30-kilowatt reactor, operating under International Atomic Energy Agency safeguards, that isn't the whole story.

The State Department says that Syria has also obtained some dual-use nuclear technologies - some with IAEA assistance - that could be used in a weapons program. Although details are murky, Russia may have agreed to assist the Syrian nuke program, too.

And don't forget about A.Q. Khan, the CEO of Pakistan's nuclear Walmart, who probably contacted Damascus during his hey-day in the 1990s. Fortunately, there is no evidence - to date - that Syria ever became a Khan client.

The possibility of Iran or Syria becoming nuclear states - or of, one coming under the other's nuclear umbrella - is a nightmare for American interests, hamstringing U.S. policy options for dealing with either problem.

The burgeoning Syrian-Iranian consortium is sowing an arc of evil and instability across the Middle East, allowing both regimes to resist international pressure on terrorism in Iraq and across the region, as well as on weapons of mass destruction.

Vigorously opposing this alliance of evil whenever - and wherever - it raises its dark, tyrannical head, is the right and necessary thing to do.

Peter Brookes is a Heritage Foundation senior fellow. His book, "A Devil's Triangle: Terrorism, WMD and Rogue States," is just out.