WASHINGTON — The Pentagon's decision to undertake the largest reorganization of its overseas U.S. troop basing in 50 years ranks as one of the most tangible accomplishments of Defense Secretary Donald Rumsfeld, who came into office pledging to shake up a hidebound bureaucracy and modernize the armed forces.

The plan would sharply reduce the number of troops stationed at bases the United States set up after World War II and maintained to fight a Cold War that ended with the demise of the Soviet Union in 1991. The Pentagon says the move — which involves shifting up to 70,000 troops in Europe and Asia to the USA over the next decade — would improve the lives of military families, make it easier to rush combat forces to global hot spots and better position the United States to fight emerging threats.

But the restructuring also entails some risk and uncertainty at a time when U.S. relations with European allies are strained and North Korea remains a threat.

Though the Pentagon won't abandon bases in South Korea, Germany and Japan, the decision to withdraw large numbers of combat troops, including two Army divisions of about 15,000 troops each from Germany, could affect those relationships in unexpected ways.

Gary Smith, executive director of the American Academy in Berlin, warns that troop reductions would mean fewer contacts between the United States and Germany, which, despite recent tensions over the war in Iraq, have been close allies for five decades. Since World War II, millions of U.S. troops and family members have rotated through Germany, fostering people-to-people relations that have helped underpin the two nations' alliance.

"Germany and America will drift apart unless there are great efforts to collaborate," Smith says.

The president officially unveiled the policy Monday in Cincinnati. Rumsfeld's plan to rethink the Pentagon's "global footprint" has been in the works for three years and widely reported for the past two.

Defense Department officials briefed reporters on the plan Monday afternoon and added more details but left many questions unanswered, including the cost and a detailed breakdown of units and countries that would be affected.

Pentagon officials outlined a number of benefits, including more frequent training with new U.S. allies such as Poland and Romania, and better positioning of U.S. troops in South Korea, who would move back from the border with North Korea.