The United States-South Korean defence relationship took a major step into the 21st century last week with the announcement that Washington would withdraw one-third of its 37,000 troops from Korea by the end of 2005. The news, not completely unexpected in the light of the U.S.'s decision in May to send 3,600 combat troops from South Korea to Iraq, still has observers on both sides of the Pacific nervous. They need not be. Even though this is the largest drawdown of American forces from Korea since the end of the Korean War -- and the most significant since 1992, when 7,000 troops left -- the reduction in 12,500 soldiers from the peninsula, viewed optimistically, is a win-win situation for America and South Korea.

First, the number of troops does not completely determine military capability. In fact, despite the decrease in American soldiers in Korea, U.S. firepower will actually increase due to expected changes in force structure over the next several years. Although technology cannot replace soldiers in some missions, today's hi-tech equipment can provide significant firepower advantages over the common foot soldier. Therefore, the U.S. can withdraw some of its Korean-based troops for other soldier-intensive missions, such as Iraq, Afghanistan and the war on terrorism, while actually improving the lethality and deterrence of its forces in Korea.

Improving the defence capability of U.S. Forces Korea (USFK) can be accomplished by bringing to bear such systems as Patriot PAC-3 surface-to-air missiles for air defence, the army's new Stryker brigade, the navy's High-Speed Vessel, and the forward-deployment of additional air and naval assets to Hawaii and Guam. Washington is also planning an $11 billion investment in some additional 150 military capabilities over the next four years that will enhance defence against any North Korean attack.

Secondly, it is useful for Seoul and Washington to reduce the visibility and "footprint" (that is, the size and number of bases) of U.S. forces because of trends in Korean public opinion, which has been mixed about USFK's presence. Moving the U.S. Army out of Seoul, drawing down troop levels and consolidating bases will reduce pressures from some sectors of Korean society for all U.S. troops to leave.

Of note, one should not conclude that the reductions in Korea will be matched in Japan. Some American troops may be shifted from Okinawa in the south to Hokkaido in the north to reduce local political tensions arising from Okinawa hosting the bulk of American forces. But bases in Japan remain critical for the defence of Japan and are more important now for other possible Northeast Asian contingencies -- including Korea.

Next, President Roh Moo Hyun early on stated his belief that South Korea should do more for its own defence. As the world's 11th largest economy, South Korea can spend more on its own defence -- and should. The reduction in U.S. forces will provide the Roh government an opportunity to do more for South Korea's national security. This supports both Washington's need for more flexibility in deploying its forces to global hot spots and Seoul's desire for a bigger role in its national defence.

Lastly, though unlikely, there's a sliver of a chance that the reduction of U.S. forces could help reduce North-South tensions. The North long has demanded that U.S. troops leave the peninsula. This reduction could be seen as a gesture of goodwill to the North that might lead to some political openings between Seoul and Pyongyang in addressing issues of national reconciliation or even the North's nuclear programme. But because no one is naive regarding North Korean intentions, the force reduction is matched by an increase in force capabilities.

The bottom line is that despite these changes, America's commitment to South Korea's defence is as strong as ever. The U.S.'s obligation to the security of the South against the North is a moral one in the defence of a fellow democracy, not to mention codified in the 1953 U.S.-South Korea Mutual Defence Treaty. The real "tripwire" is the treaty, not the number of U.S. troops in South Korea.

The alliance has successfully deterred North Korean aggression for over 50 years. It will do so for as long as needed. A military confrontation between the North and the South would invariably result in the demise of the regime in Pyongyang. Fortunately, Kim Jong Il understands this. Adjusting the U.S.-South Korea partnership for the 21st century makes ultimate sense. The future of the alliance will be better for this, making the relationship ready for challenges on the Korean peninsula and beyond.

Peter Brookes, a veteran of the CIA and naval intelligence, is a senior fellow at  The Heritage Foundation.