US strategy and forces don't match, planners say. With budgets tight, strategists disagree on whether US military forces should be enlarged, or US commitments reduced.

Within the Pentagon's vast expanse, the US military is preparing uneasily for the post-Reagan era. A budget crunch is officers' most immediate concern. But many also worry about a broader, long-term problem: Call it the ``strategy gap.''

This gap is the shortfall between the size of US armed forces and the number of commitments that the United States has made to police areas of the world. Pentagon officials have long said American power is spread too thinly around the globe - and with force size shrinking because of lack of cash, they say this problem is becoming acute.

In peacetime, the strategy gap means such things as lengthening periods at sea for aircraft carriers, and few troops behind the US pledge to defend the Persian Gulf. In a crisis it could mean much more.

``We couldn't possibly fulfill all the requirements for a global conflict,'' said Air Force chief of staff Larry D. Welch in an interview earlier this year. ``None of the other services could, either.''

The Pentagon's prescription for the problem is no surprise: more money for more people and weapons, or failing that, at least enough to keep current forces from getting smaller. But some military officers and a number of experts outside government are focusing more and more on commitments, the other side of the equation.

Does the US have to be the world's policeman, especially in light of the warming trend in relations with the Soviet Union?

``Our global commitments have not been thought through coherently,'' said Col. Stuart Perkins, a former strategist with the Joint Chiefs of Staff (JCS).

Since the official strategy of the US is ``forward defense'' - that is, to protect the continental US by establishing front lines overseas - about one-quarter of all active-duty US uniformed personnel are based on foreign soil. Installations range from the big Army and Air Force facilities in Western Europe to a small Navy port of call on the West Indies island of Antigua.

For 40 years, the main purpose of this defense infrastructure has been to prevent the Soviet Union from expanding its influence through invasion or intimidation. Since the US defense cordon makes a rough half circle around the Warsaw Pact, strategists refer to ``containment'' of the USSR.

US forces are supposed to be able to resist a Soviet move at any point along this containment cordon. Theoretically, they also have the capability to counterpunch. Defense officials say that if the Soviets invaded Western Europe, the US might strike back in the Far East. ``We want to face them with the uncertainty of a two-front war,'' said a senior defense official who requested anonymity.

But the Pentagon also admits that there are gaps in the cordon. In the awful contingency of another global conflict, the US would have to fight in major theaters of war in sequence, rather than simultaneously.

``The military wasn't big enough in World War II, even with mobilization. I don't think it's any different today,'' said the Army chief of staff, Gen. Carl Vuono, earlier this year in one of a series of interviews with officials on US strategy.

Joint Chiefs of Staff planning documents - derided as ``wish lists'' by critics - say the US needs 19 aircraft carriers to fight a global war, instead of the planned force of 15. The Air Force needs more than 60 tactical fighter wings under these plans, as opposed to today's active and reserve total of 38.

The Persian Gulf is probably the area where the US defense commitment is thinnest. The military command charged with responsibility for the Gulf region, the US Central Command, on paper has approximately four Army divisions to call its own. But many of the units that make up this force are also pledged to the defense of Europe. Since no Gulf country will grant the US rights to a permanent base, the Central Command has had to make do with a delicate network of pre-positioned equipment sites in Oman and other nearby nations.

``The difficulty ... of going out there and defending that just tears the eyes,'' Colonel Perkins, the former JCS strategist, said.

The fall in the price of oil has made the Gulf less important to the US, some defense officials note - meaning that perhaps the US could afford to scale back its commitment. ``The Persian Gulf can only get us in trouble,'' said one Pentagon strategist who asked to remain nameless.

The Far East is one area where the US has in the past tried to scale back - and failed. In 1977 President Carter announced a pullout of the 32,000 US ground forces based in South Korea. The move was scuttled by adamant opposition from within the government and by the Soviet Afghan invasion.

Europe, home to 350,000 US personnel, has long been the focus of US defense efforts. For almost as long, Congress has periodically considered whether to reduce the US presence there. Though it's unlikely that troop cuts will be ordered this year, increasing budget problems could mean more serious consideration of such reductions in the near future. A House Armed Services panel recently issued a report calling for US allies to pay more for the common defense. ``Our allies are not sufficiently aware of the strong policy pressure in this country to reduce our defense commitments,'' the report said.

Military officers say there are areas where they might be able to reduce peacetime operations. Adm. Carlisle Trost, chief of naval operations, said he might be able to reduce the Navy's presence in the Mediterranean and, now that the Seoul Olympics are over, in the Western Pacific.

But he adds that ``when you withdraw from somewhere, you don't have much influence in what happens there. And as long as we're a superpower, we're going to want to maintain various levels of influence around the world.''