YONGSAN GARRISON, South Korea — The threat from North Korea hasn't suddenly changed. It claims to have nukes and its million-man army is ready to roll. China, meanwhile, is emerging as the new Asian military leader, and terrorism is flaring up all over the region.

But at the United States' major Asian outposts, some serious downsizing is under way.

In Japan and South Korea, two countries President Bush visited last week, a U.S. military presence that had remained relatively static for years is undergoing a huge change. In Washington's two primary Asian partners, troops are being pulled out by the thousands, and well-established bases shut for good.

The American position isn't weakening, officials and analysts say; the cutbacks will be counterbalanced by improved equipment, organization and cooperation.

"We have always been unique and we have always maintained the old-style structure and doctrine," said Col. Richard Parker, chief of force development and plans for the 8th U.S. Army, headquartered in the heart of Seoul, just 30 miles from North Korea.

Not anymore.

In its biggest reorganization in two decades, the U.S. will shed 12,500 of its 32,500-strong force in Korea over the next three years, reduce its number of bases by about three quarters and hand over major elements of the troops' mission to their Korean counterparts, who will "play a larger and larger role," U.S. Defense Secretary Donald Rumsfeld said on a recent Asia tour.

Similar restructuring is afoot in Japan, where nearly 50,000 U.S. troops are stationed.

The United States and Japan have just agreed to the most sweeping changes in the deployments there in recent memory, a plan that — along with basing a nuclear-powered aircraft carrier just south of Tokyo — includes the withdrawal of about 7,000 of the 18,000 Marines on the crowded island of Okinawa.

Sheila Smith, an analyst with the private East-West Center in Honolulu, says the aim is to streamline, but not undermine, the alliance.

"Rather than planning for fixed contingencies, such as the defense of an ally, the U.S. Department of Defense would like to retain flexibility over the deployment of its military, and access to facilities and bases around the globe," she wrote recently. "Japan has embraced these priorities for its own military, and so the alliance now places a premium on joint operations and the interoperability of the (Japanese military) and U.S. forces in the region."

In Korea, the 8th Army is scattered across 41 major bases and several dozen smaller camps. "Eventually, we'll be down to about eight," Parker said.

The changes in Korea are in line with shifts taking place within the entire Army, moving toward combat teams that are "smaller but fully capable and fully lethal packages that can be deployed faster," Parker said.

Yongsan Garrison, a Seoul landmark, will be moved, along with overall U.S. military headquarters, to Pyongtaek, 40 miles south of the capital, by 2008.

By the end of this year, the 8th Army will have shed 8,000 troops. Another 3,500 will leave by 2008, along with 1,000 Air Force personnel.

Parker stressed that in conjunction with upgrades in technology, forces here will be as capable as ever of deterring North Korean attack.

Facing increasing demands on its own troops in Iraq and elsewhere, Washington is pushing Seoul and Tokyo to assume a bigger role in regional security and in their own defense — and both appear willing.

The two governments are also under pressure from communities near the bases to scale back the U.S. presence, and from left-leaning groups that want to cut the alliance altogether.