Washington ? In a historic move after a half-century, the United States will pull its ground troops away from the Demilitarized Zone separating North and South Korea and consolidate them at bases well to the south.

The realignment, announced in a joint U.S.-South Korean statement Thursday, has been in the works for months. It reflects Defense Secretary Donald H. Rumsfeld’s push to break a Cold War mold of assumptions about the usefulness of having troops along the tense DMZ.

The moves in Korea are part of a broader Pentagon strategy to realign U.S. forces around the globe, to include likely reductions in Germany and the establishment of new bases in eastern Europe. Last month the United States pulled its troops out of Saudi Arabia after a 12-year stay.

In Rumsfeld’s view, moving U.S. troops away from the DMZ and consolidating them at perhaps two main “hub” bases south of Seoul will create a more formidable fighting force. It will not remove the troops from danger, and in Rumsfeld’s view it does not weaken the U.S. deterrent at a time when communist North Korea is openly pursuing an expanded nuclear arsenal.

The United States has assured South Korea it will spend more than $11 billion over the next four years on 150 improvements in the combined U.S.-South Korean defenses. No details were provided.

The new arrangement will end a U.S. troop presence on the DMZ that dates to the end of the 1950-53 Korean War, when a 151-mile buffer zone was established along the approximate line of ground contact between the opposing forces at the time a July 27, 1953, truce was signed.

In a two-phase movement, U.S. troops at bases scattered near the DMZ will be moved to “hub bases” at least 75 miles south of the Demilitarized Zone, according to the joint statement.

Even after the consolidation, however, U.S. troops will rotate to training ranges close to the DMZ.

Officials gave no timetable for the withdrawal, reflecting persistent South Korean worries that any reductions would put it at greater risk of a North Korean attack. Most troops at the 8th U.S. Army headquarters in Seoul, 37 miles south of the border, also will move south.

Paul Wolfowitz, the deputy U.S. defense secretary, previewed the announcement during a visit to South Korea this week.

“The essence of what we’re trying to do is to make sure that the forces we have here on the peninsula can respond quickly and immediately, even before reinforcements arrive, if there were ever to be an attack,” he told a Seoul news conference Monday. “And I think the ability to do so is something that will not only strengthen deterrence but save lives in the horrible event that a war should occur.”

The main U.S. Army force in Korea is the 2nd Infantry Division, headquartered at Camp Red Cloud north of Seoul.

The U.S. pledge to conduct exercises near the DMZ “will mean that U.S. troops will continue to play the role of a tripwire to deter war,” said South Korean Assistant Defense Minister for Policy Lt. Gen. Cha Young-koo, who led the South Korean side in talks. The Americans were led by Richard Lawless, the deputy assistant secretary of defense for Asia-Pacific Affairs.

Rumsfeld wants to give the U.S. forces in Korea the flexibility to train for missions elsewhere in the region. This will be facilitated by having most of them consolidated at hubs like the Osan air base south of Seoul and the Chinhae and Taegu areas in the southeast.

Some have argued that the United States should not pull troops away from the border area unless the North Koreans reciprocate, but Rumsfeld sees that as outdated thinking. He believes the U.S. moves should be made regardless because they strengthen South Korea’s defense, not weaken it.

South Korean President Roh Moo-hyun has said the withdrawal of U.S. troops should be a bargaining chip in any talks with North Korea on reducing its massive troop deployment along the border.