The massive deployment of U.S. forces to the Persian Gulf has called into question whether the United States military, even without a Soviet threat, can shrink as mandated by Congress and still project enough force in times of international crisis to confront well-armed foes.

In the defense authorization bill enacted last month, Congress instructed the Army, Navy, Air Force and Marine Corps to prepare for fighting limited wars in distant countries rather than expect to slug it out with Soviet divisions in Europe. At the same time, Congress ordered deep cuts in the active-duty manpower base from which the services will have to build the lighter, faster, more lethal forces envisioned for the 1990s.

"Given the nature of the multi-polar war we face," retired general John A. Wickham Jr., former Army chief of staff, said in an interview, "Congress should be rather cautious in setting end strengths" for active-duty forces. He added that once today's high-quality forces are reduced, they cannot be built back up quickly with the same caliber of personnel.

Wickham's warning typifies the uneasiness many military professionals are feeling as Congress cuts manpower and funding while mandating a difficult and expensive restructuring of today's forces. They cite the current troop deployment in the gulf at a cost of over $2 billion a month as an example of how much it takes to combat the aggression of even a small country armed with modern weapons.

The gulf deployment has not gone the way Congress imagined military emergencies in the post-Cold War era. The buildup has been slow, featured heavy armor and, except for the Marines, required massive resupply from stockpiles in the United States.

Moreover, the Joint Chiefs of Staff has not had enough confidence in combat reservists to activate a sufficient number to supplement the active-duty brigades sent to the gulf.

President Bush, in sending more than 400,000 U.S. troops to the gulf, will be drawing from an active-duty manpower base of about 2 million, already one-third smaller than the 3 million President Richard M. Nixon had at his disposal in 1970 when the United States had about the same size force of soldiers, sailors, airmen and Marines in Vietnam.

Nixon also had the benefit of a draft to provide troops for European billets or replace men killed and wounded in Vietnam.

The number of active U.S. troops is scheduled to diminish further under the new authorization bill, falling to 1,976,405 in fiscal 1991 and to 1,613,000 in fiscal 1995.

"The Warsaw Pact is no longer a credible military threat," said Congress in the fiscal 1991 defense authorization bill signed by Bush last month. "The active military forces will be significantly reduced. . . . The Department of Defense should shift a greater share of force structure and budgetary resources to the reserve components of the armed forces . . . "

While scaling down the U.S. military, defense planners face a choice between withdrawing into a kind of isolationist "Fortress America" or creating a more flexible, rapid deployment fighting force. Congress is pushing for the latter option.

"To meet potential force projection missions, the U.S. must restructure its forces," the Senate Armed Services Committee said in reporting out its version of the fiscal 1991 defense authorization bill. Top priority should go to "forces that are inherently mobile and rapidly deployable: maritime-based expeditionary forces, long-range and tactical air forces and light forces . . . "

Sen. William S. Cohen (Maine), ranking Republican on the Senate Armed Services subcommittee on projection forces and regional defense, explained the rationale for such restructuring this way: "We shouldn't become a prisoner of world events, but you simply can't board up the Pentagon and walk away from the world. The world isn't going to walk away from us. We have to restructure our forces to take into account what are the likely threats that face us in the future."

If "you shrink back to a Fortress America," Cohen added, "then you're locked into strategic systems like the MX missile or B-2 bomber which are very expensive and very unlikely ever to be used."

The House Armed Services Committee, in drafting its authorization bill this year, also called for restructuring to give the armed services a longer, quicker reach in the post-Cold War world.

But Chairman Les Aspin (D-Wis.) said that while House members know there must be a new role for the U.S. military in the 1990s, they have not decided what that role should be, other than the general conviction that forces be smaller, lighter, faster and more lethal so they can reach a world troublespot in a hurry and have enough punch to influence outcome of events there.

Congress has asked the Pentagon to submit reports next year detailing "the total mix of airlift, sealift, amphibious lift, surface transportation and prepositioned war material -- both at sea and on land -- necessary for the United States to respond to contingent threats against the national security interests of the United States during the remainder of the current decade and beyond . . . Operation Desert Shield deployments {in the gulf region} shall be included among the scenarios examined . . . "

Where tomorrow's troops for the next distant crisis should come from; how the Pentagon budget should be divided between active and reserve forces; whether the United States needs both a light Marine Corps and a light Army -- all are among the questions that the end of the Cold War and the Persian Gulf crisis have brought to the fore, underscoring the stresses and strains the United States confronts in trying to play world policeman.