The United States and Japan agreed yesterday to move forward with the biggest overhaul of the Pacific alliance in decades, aimed at bolstering military cooperation against new threats while consolidating U.S. forces on the island and withdrawing about 7,000 Marines from Okinawa.

The Marine ground and air forces, including the headquarters of the Third Marine Expeditionary Force, will be transferred to Guam to build up forces there. The move, expected to be complete in six years, will reduce the number of Marines on the island of Okinawa from 18,000 to about 11,000, defense officials said. Japan agreed to work with the United States to finance and thereby accelerate the move to Guam, in part to alleviate long-standing Japanese frictions with American forces on Okinawa.

The plan's goal, outlined in a 14-page bilateral report released yesterday, is for Japan -- with U.S. backing -- to beef up its defenses against threats ranging from ballistic missiles to attacks by guerrilla forces, or an invasion of its remote islands. Meanwhile, the militaries will realign their forces in Japan so they can work together more closely to counter regional and global threats such as terrorism.

"This relationship must and is in fact evolving to remain strong and relevant," said Defense Secretary Donald H. Rumsfeld at a news conference after meetings with his Japanese counterpart Yoshinori Ono, Secretary of State Condoleezza Rice and Japanese Foreign Minister Nobutaka Machimura. The plan is part of the Pentagon's review of the posture of U.S. forces around the globe.

"We are in fact opening a new era" in the alliance, Ono told reporters.

A subtext of the cooperation, hinted at in the report, is to counter China's military buildup, which the Pentagon views as a growing threat not only to Taiwan but also to other Asian countries, such as Japan and India. Close defense cooperation "is essential to dissuade destabilizing military buildups, to deter aggression and to respond to diverse security challenges," the report said, in an apparent reference to Beijing's rapid military modernization. It called the U.S.-Japan alliance the "anchor" of regional security.

The plan has two major thrusts -- strengthened bilateral military cooperation and a realignment of forces and bases on Japan, where about 37,000 U.S. troops are currently stationed. Washington seeks to finalize all agreements related to implementing the plan by March 2006, although many steps will take years to execute, according to senior U.S. administration officials.

"This is really the first major readjustment in the alliance across the board," said a senior administration official who spoke on background in advance of the report's release. It specified areas that would receive special focus in the new cooperation, including air defense, minesweeping and maritime interdiction, and ballistic missile defense.

Close coordination and "high readiness" is especially vital in missile defense, the report said, "in light of the minimal time available to respond to a ballistic missile threat." The United States will deploy an X-Band radar system to Japan to help intercept missiles directed at the island -- a development acknowledged for the first time in yesterday's report, said a senior U.S. defense official. It will also deploy to Japan "as appropriate" Patriot PAC-3 and SM-3 missile defenses, the report said.

U.S. and Japanese militaries will intensify planning for contingencies, deepen intelligence sharing, and expand joint training inside and outside Japan, where a dense population constrains military use of land and airspace. Bilateral training and exercises will be more frequent on the U.S. mainland as well as Hawaii, Alaska, and especially Guam.

The report calls for the two forces to share bases on Japan to increase their ability to conduct military operations.

Major aspects of the realignment of forces include the Marine move to Guam, which will allow for the consolidation of Marine forces and return to Japan the use of "significant land," the report said.

The United States is willing to discuss with Japan "a much larger move" to the north of existing facilities in Okinawa, where 75 percent of the U.S. military's exclusive use areas in Japan are located, a defense official said.

The United States will establish an operations center with Japanese forces at Yokota Air Base, where the Japanese air defense command will be set side by side with the U.S. 5th Air Force, to strengthen coordination and data-sharing between the countries' air and missile defense commands. The U.S. Army will move 300 soldiers from Fort Lewis, Wash., to Camp Zama, Japan, for a deployable headquarters there.