South Korean President Roh Moo-hyun meets in Washington today with President Bush at a time of great tension and increasing uncertainty in their nations' alliance.

The persistent source of friction is fundamentally different approaches toward North Korea generally and its nuclear weapons program in particular. South Koreans see their northern cousins as more to be pitied than feared. The South's "peace and prosperity" policy of engaging the North is predicated on postponing reunification of the two Koreas until far into the future, a view shared by all the other countries in the region. Meanwhile, Seoul prefers to tease the North out of its isolation, tame its belligerent ways and gradually introduce market economics to raise up a destitute population.

Washington sees this approach as underwriting an evil regime that brutalizes its own people and continues to threaten its neighbors. The Bush administration believes that regime change, preferably sooner rather than later, is the only way to bring true peace and prosperity to the Korean peninsula. In its view, the North's recent test-firing of seven ballistic missiles was just the latest indication that Seoul's engagement policy has failed to curb Pyongyang's behavior and make the region safer.

Other issues also cloud the relationship, including friction concerning a free-trade agreement, a reconfiguration of the roles and missions of the U.S. military in South Korea and the perceived lack of American appreciation for South Korea's significant contribution of troops to Iraq. Moreover, a self-confident new generation of South Koreans doesn't recall the shared sacrifices of the Korean War and doesn't reflexively defer to the United States. The alliance is in worse shape today than only a few years ago.

The value of the U.S.-South Korean alliance is more important than differences about how to deal with North Korea. For the time being, Bush and Roh should agree to disagree on North Korea and move onward.

Traditionally, countries ally because they share common threats, values or interests; ideally, they should share all three. Alliances provide mutual security, formally binding together military and diplomatic institutions in predictable, nonthreatening ways. Adopting a common stand against North Korea, by itself, no longer provides enough "glue" to bind the U.S. and South Korea together. But powerful reasons for future cooperation, grounded in national self-interest, still exist.

Geography alone demands that the two countries work together. A rising China, with growing economic and military power, presents new concerns for Seoul and Washington. Historically, China has claimed Korea as part of its traditional sphere of influence. The South Korean business community is increasingly worried about Chinese competition hollowing out its steel and other heavy industries. In a recent poll, 38 percent of South Koreans identified China, not North Korea, as posing the greatest threat to the South in 10 years.

For the U.S., the partnership with South Korea provides bases on the east Asian mainland, taking pressure off the U.S.-Japan alliance. It helps reassure Seoul of Japan's peaceful intentions as Tokyo develops a more "normal" military and security role in the region. And it's a necessary precondition for any larger efforts that the U.S. and its allies might undertake to strengthen multilateral security cooperation among democracies in the region.

Beyond this, the U.S.-South Korean alliance gives Seoul a direct channel to Washington and American friendship. Alliances do not just provide standby capacities for mutual defense; they create ongoing political relationships and mechanisms for doing a wider array of business. This political architecture will become more important — not less — in the decades ahead, regardless of the fate of North Korea.

Below the level of great-power politics, a host of transnational issues plague northeast Asia, such as environmental pollution, narcotics and human trafficking, piracy and counterfeiting. No one country can adequately tackle these challenges; cooperation among Seoul, Washington, Tokyo and others is essential.

The U.S.-South Korean alliance will never be perfect; no alliance is. If its original rationale has changed over the decades, there remain important reasons for Bush and Roh to re-imagine and revitalize it.

G. John Ikenberry is a professor of political science at Princeton University and the author of "Liberal Order and Imperial Ambition: Essays on American Power and International Order." Mitchell B. Reiss is vice provost of international affairs at the College of William & Mary. They wrote this commentary for the Los Angeles Times.