It’s been nearly a decade since the leaders of South Korea and the United States have been on the same wavelength. Kim Dae Jung and Bill Clinton saw eye to eye on North Korea. But the Nobel Prize-winning Kim and George W. Bush had an infamously testy relationship. And Bush and Roh Moo-Hyun were not exactly political bedfellows, though the latter leaned over backwards to curry U.S. favor with a free trade agreement and deployments of Korean troops to Iraq and Afghanistan. True, Bush and Lee Myung-bak had a brief and better relationship, but the beef deal they cut nearly ended Lee’s term before it had properly begun.

The two current leaders in Seoul and Washington have converged in their policies not so much because of clever diplomacy or inherent similarities of character. Rather, North Korea, through its reckless behavior, has played the role of matchmaker. By testing a nuclear weapon and pulling out of the Six Party Talks, North Korea has helped narrow the policy and perception gap between the United States and South Korea.

So, for instance, both South Korea and the United States pushed hard for a strong resolution at the United Nations that adds further sanctions and strengthens the interdiction regime against North Korea. Both leaders have reiterated that they don’t intend to reward North Korea for its bad behavior. And the United States has restated its alliance commitments to South Korea, which include the maintenance of the nuclear umbrella.

Don’t be fooled by the cordial words and coordinated policies. There is still considerable daylight between the positions of South Korea and the United States. The two countries have fundamentally different relations with North Korea. Washington is focused exclusively on the nuclear issue. U.S. negotiators have sat down bilaterally with their North Korean counterparts to hammer out rough agreements that they then brought to multilateral discussions that include South Korea. And the Obama administration, distracted by so many major foreign policy crises not to mention a domestic economic mess, has so far treated North Korea as a rather low priority.

South Korea, on the other hand, doesn’t have that luxury. Seoul can’t ignore Pyongyang. Nor is the nuclear issue South Korea’s only focal point — conservatives worry about North Korea’s conventional capabilities while engagement advocates stress the economic ties of the Kaesong industrial complex. And South Korea has to struggle with the fact that the North considers negotiations with the United States a greater priority.

The two countries currently enjoy unanimity in their policy toward North Korea only because the latter has taken such a hard-line stance. Once Pyongyang realizes that it can deal with Obama more easily than with Lee, the consensus reached in Washington this week will begin to break down (though of course the rhetoric of alliance will remain strong). The United States will go back to face-to-face meetings with Pyongyang, which Obama supported as a candidate, and South Korea will return to a peripheral role. And if Lee the pragmatist manages to save the Kaesong project — and push forward on his larger economic engagement program — South Korea will again focus more on improving the lives of North Koreans than on removing the nuclear threat.

Of course, North Korea was not the only issue at the summit. But the two countries can reach even less agreement on the other items on the agenda. The free trade agreement is at an impasse with neither side willing to make major concessions. There is still tension around the issue of the transfer of wartime military control, planned for 2012. And there might be a difference of approach in how the two countries are dealing with their citizens currently held in North Korea: Lee has appealed to the United States to raise the issue of the Kaesong complex employee along with the two American journalists, but Obama has yet to talk about the latter publicly.

This summit, like most high-level confabs, aimed for consensus. The two leaders tried to speak with one voice, at least on North Korea. When Kim Dae Jung and Bill Clinton both attempted engagement policies with North Korea in 2000, the results were quite good. Perhaps a stern, coordinated response this time around will have a similarly positive effect, forcing North Korea back to the table.

But this unanimity may also backfire. The agreement on “extended deterrence,” which commits the United States to retaliate with nuclear weapons if North Korea attacks the South, is yet another step in an escalating conflict. North Korea, with so little to lose, is the master of brinksmanship. It is not wise to enter into a tit-for-tat match with such a country.

At this point, more important than finding common ground between the United States and South Korea is establishing common ground between North Korea and the rest of the world. By all means, Washington and Seoul should coordinate policy. But they should also keep their eyes on the prize: resolving the current crisis with North Korea without resorting to force.