Marines come pouring in from Okinawa. B-52 bombers fly over from Guam. There is an infantry task force from Hawaii, a Lance missile group from Oklahoma, a Special Forces unit from North Carolina, a naval task force from the 7th Fleet, and some National Guardsmen from Utah.

With this unusually large deployment from abroad, U.S. military forces are engaged in a highly visible demonstration of the proposition that they can help defend South Korean after American ground forces are withdrawn beginning late this year.

They are part of "Team Spirit '78" which military officials say is the largest peacetime exercise ever staged by American forces overseas.

It is the third in a series of Joint U.S.-South Korean exercises planned several years ago but it has taken on a special significance this year because of the impending withdrawal of American ground forces.

According to the schedule announced last year, all U.S. ground forces are to leave here no later than the end of 1982. The only U.S. contingents to remain will be one Air Force and two Marine squadrons and an Army intelligence unit.

The large infusion of troops and aircraft from distant points is designed to reassure the South Koreans and Japanese that despite the withdrawal American forces can rush back into the country in time to help deter a sudden invasion by North Korean armies.

Normally, the exercises are closed to the press but the one this year has been thrown wide open to assure maximum publicity and more than 300 reporters from South Korea, Japan, Western Europe and the United States are observing.

The Pentagon dispatched Deputy Defense Secretary Charles W. Duncan to inspect the exercise and to repeat American military promises in talks with government leaders in Seoul and Tokyo.

Duncan, at a news conference Tuesday, proclaimed the exercise "a clear demonstration of our ability to rapidly augment forces in Korea" and a way of proving the ability of U.S. and South Korean forces to operate together under near combat conditions.

The South Koreans grimly accept the projected withdrawal of some 30,000 U.S. ground forces as inevitable but are skeptical of the American claim that it does not affect the commitment to deter North Korean aggression.

They also believe that the pull-out may encourage the enemy to strike and that the likelihood of war is increased.

That view is shared by the American commanders here who also believe that a war would be a disaster for South Korea even if it won.

The reason is geography. This capital city, with about a fifth of the total population and much of the country's economic power and government facilities, sits only 25 miles from the Demilitarized Zone where North Korean armies are massed. In a sudden surprise attack, it is feared Seoul could be pulverized and occupied by enemy forces in a matter of hours.

The military strategy has been devised around a calculated gamble that Seoul can and must be defended and until last year it was assumed that the U.S. 2nd Division would be part of this so-called "forward defense" stragedy. The first units of that division will be pulled out later this year under a schedule adopted by the Carter administration.

To fill in that hole, military planners are counting on a rapid deployment of U.S. forces from bases elsewhere and that is what "Team Spirit" is all about this year.

It assumes that a combination of Marines from Okinawa, infantry from Hawaii, and other units from the continental United States could be shuttled in quickly and then set into the battle for Seoul. U.S. aircraft would be flown in to intercept North Korean planes and to smash the formidable North Korean tank forces.

In the current 11-day exercise, about 115,000 troops are staging landings on beaches, river-crossings, and other maneuvers. Some 22,000 troops from overseas bases have joined 20,000 American and 73,000 South Korean troops stationed in the country now. In addition to about 200 aircraft involved, an amphibious force backed by the carrier Midway is deployed on the coast.

The large-scale operation has prompted an unusually strident denunciation by North Korea. Its Communist Party newspaper, Rodong Sinmun, called it a "grave provocative action" designed to drive the South Korean government into a "fresh military adventure" on the peninsula.

The presence in the exercise of a Lance missile battalion from Fort Sill, Okla., has drawn extensive press coverage by foreign reporters. The Lance is a ground-to-ground missile that can be equipped as either a nuclear or conventional weapon. Its deployment here has raised again the question of whether the U.S. forces would use nuclear weapons to defend South Korea.

As usual, military spokesmen refuse to comment, saying only that those Lances deployed here now are not nuclear.

Deputy Defense Secretary Duncan, when asked the question, said only, "It is the long-standing policy of our government that we neither confirm nor deny the presence or non-presence of nuclear weapons." He added, "We are confident that we can defend Korea in a conventional way."