When it came to Communist aggression, President Truman drew a line in the sand. And on June 25, 1950, North Korea crossed it. Truman ordered U.S. troops into battle. In 1953, an armistice ended the bitter conflict, but relations between the U.S. and North Korea -- not to mention those between the two Koreas -- have been tense ever since.
The nature of the conflict has changed, however. North Korea is no longer bent on conquest. Today, Pyongyang has two goals. One is keeping a small elite in power and living a life of luxury.
The other is blackmailing the world into sending enough food aid to keep the rest of the country from starving to death and thus living to service the elite.
What makes North Korea so troublesome is the method by which it continues to command the world's attention. Pyongyang employs a combination of intimidation via nuclear weapons and outright armed attacks on the South.
Recent unprovoked attacks include last year's March sinking of a South Korean naval vessel and the November shelling of Yeonpyeong Island.
President Obama took office promising to reach out to America's adversaries, and so he did. But North Korea promptly blew off his make-nice overtures and continued its tactics of intimidation. Despite the North's recalcitrance and its military provocations, Obama has drawn no lines in the sand.
Most Korean experts expect another round of provocation from North Korea this summer. Pyongyang calculates that "minor" misbehavior leads countries to give it more food aid out of fear that, if they don't, Pyongyang will do something even more desperate.
But there's a second reason the North may view outrageous behavior as particularly productive this summer. It might influence elections in the South.
Pyongyang may calculate that the threat of war will prompt voters in the South to prefer a new government that will take a less hard line with the North.
The experts also expect that the next North Korean provocation will be creative, something they haven't tried before. More worrisome, they predict that Seoul won't take the next attack sitting down. The government will have to respond with more than just harsh words.
Nor is it unlikely that North Korea will conduct another long-range missile test. Missile tests are aimed at sending a message to Japan and the United States, not Seoul. It's a not-so-subtle way of saying, "Don't mess with me when I mess with South Korea."
To really grab Washington's attention, the North might even throw in a nuclear test -- maybe this time with a uranium-based weapon (something they have not demonstrated before).
The forecast, then, is a summer of troublemaking in the Land of the Morning Calm. And that could make for a long hot summer for Obama. The White House would like to keep foreign policy and military matters on the back burner all summer. The only defense topic the president wants to discuss is how much more the Pentagon's budget can be cut.
The last thing Obama wants or needs is a summer crisis in Korea. Both Pyongyang and Seoul know that. They also know the polls showing Americans are more demoralized about foreign policy than ever before.
All this will be considered as the North Koreans calculate their next moves. The problem is their math may add up to something that turns out really bad.
Obama should do more than just hope that doesn't happen. Letting both the South and the North know now that the U.S. is serious about backing up Seoul next time is the best step he could take.
James Jay Carafano is a senior research fellow for national security at the Heritage Foundation.