After a post-Vietnam-war decade of withdrawal, the United States is again seeking military outposts in distant lands, but in a vastly different and more cautious fashion than in the past.

In sharp contrast to the globe-girdling string of huge U.S.-built and - operated bases built since World War II from Western Europe to the Philippines, the new approach is symbolized by delicate agreements with three countries bordering on the Indian Ocean, Oman, Kenya and Somalia.

United those agreements, which administration officials say should be made final in a month or so, the United States would not build bases. Rather, the three countries would allow expanded access to their existing port and airfield "facilities" by U.S. ships and planes.

The idea is to give the United States at least some ability to shift the strategic balance with the Soviet Union by being able to operate and sustain military forces near one of the world's most volatile and economically crucial regions, the oil-rich Persion Gulf.

The new arrangements tend to be viewed publicly against the backdrop of current turmoil in that region. But in a broader sense, they reflect the first real reversal of American basing policy since Vietnam, and could set a pattern for similar arrangements elsewhere.

The move represents a major political diplomatic and military gamble for the United States and those nations in the area that publicly or privately support it.

Specialists say the real test will be whether the United States can adroitly manage an increase in military presence while keeping a low profile. The idea is to help deter Soviet inroads and ease fears of conservative Arab states, such as Saudi Arabia, about the Soviet threat.

This must be done, however, without stirring internal instabilities and subversion within weak but friendly local governments that could wipe out potential military gains.

The Carter administration obviously sees more pluses than minuses in these new arrangements, and recent interviews in several government agencies yield the following balance sheet.

On the military side, officials say the United States clearly will have to live with a lot more ambiguity and contingencies in using these facilitiies than it does operating out of big, established U.S. bases elsewhere.

On the other hand, they say access to several different facilities, adequate for some needs, may be better than having a single big base in so volatile a region.

Some top military planners say the big-base era may be over, with Congress, recalling the huge loss of U.S. material and bases in Vietnam, unlikely to approve new ones in places like the gulf.

The Soviets have been stung in the same fashion, being kicked out of Somalia after investing millions in airfield and port facilities there.

There is noplace within the Arab or Moslem world surrounding the gulf where the United States could build a big base if it wanted to, officials say, largely because of the unresolved Palestinian problem, and, to a lesser extent, the problem of divided Jerusalem.

The failure to settle these symptoms of Arab-Israeli bitterness is a major factor complicating U.S. strategy in the gulf, affecting many things, from Saudi Arabia's inability to be more openly friendly to the United States to negotiating overflight rights.

The most remarkable diplomatic achievement in negotiating the three news agreements may be that it gives the United States some military foothold in the region without an Egyptian-Israeli accord on the Palestinian question.

Tiny Omam, with its port at Muscat and air base on Masira Island, offshore, it strategically the most important of the three new arrangements. It is located right at the mouth of the Persian Gulf, near the Straits of Hormuz, through which the West's oil tankers pass.

Planners say a major benefit would accrue to U.S. strategy rather quickly with increased ability, for example, to land Navy reconnaissance and antisubmarine warfare patrol planes there for refueling or repair rather than having to fly 4,000 miles to the big U.S. airfields in the Phillippines. This would also increase the time on patrol in the gulf area.

The Oman facilities, they say, will also enable transport planes from the United States to land, carrying everthing from equipment to mail, much closer to the U.S. fleet. The material would then be ferried to the fleet. cThe material would then be ferried to the fleet by smaller ships or aircraft.

Soon, 1,800 Marines in a four-ship amphibious task force will join the rest of the fleet in the Arabian Sea, and planners are also hoping that the new arrangements, especially with Oman and Kenya, will be places for shore leave.

The United States is investing heavily in strengthening runways and expanding facilities on Diego Garcia, a tiny island in the Indian Ocean owned by the British and used as a way station by U.S. vessels en route from the Pacific. Diego Garcia, though important, is still 2,700 miles from the gulf.

The port and long runways at Berbera in Somalia are about 1,300 miles from the entrance to the gulf. While more distant than Oman, Somalia is close to Saudi Arabia and some planners say the ability to operate close to the Saudis from both places will be a plus for both the United States and Saudi Arabia.

Mombasa, the Kenyan port, is almost as far away from the gulf as is Diego Garcia, but planners say that, aside from providing still another alternate route in the region for U.S. forces, the United States could benefit simply from increased ties with an influential African nation that is going in what the United States considers "the right direction."

At the moment, the United States -- with the exception of offshore naval power -- is at a clear disadvantage militarily in the gulf with Soviet land and air forces that are much closer and more numerous.

Ultimately, however, the United States plans to have a 100,000-man or more rapid deployment force that could be dispatched quickly into trouble spots, such as the gulf, by sea and air. When new ships and planes to carry that force are built and put into operation during the next several years, the facilities in the three countries and possibly others where discussions are going on. Would play a key role as transit points.

Officials claim, however, that there is no plan to actually stockpile prepositioned arsenals of tanks and guns in these countries to be picked up by troops flown in from the United States. Rather, they say, the plan is to store all the major equipment on ships off-shore that are able to move around as needed.

The ports and airfields would be used as places to land, transfer cargo, take on fuel, water and food, get an occasional spare part and rest.

The U.S. fleet is no stranger to these countries. There have been more than a dozen U.S. ships sailing into Oman ports in the past few years, with less frequent visits to Mombasa and Berbera.

Eventually, a few hundred U.S. servicemen are likely to be stationed in the three countries to handle the liaison with U.S. forces.The United States also will be making repairs to various port and runway facilities, plus making sure that certain types of fuel is available and can be stored there.

In addition, however, the United States also will be providing economic and military aid to all three countries in return for their willingness to provide facilities. Officials are exptected to tell Congress this week how much that should be. State Department officials insist that the amounts will be modest, in part because it would be difficult for these relatively small nations to absorb a large increase.

It is the prospect of supplying American arms to Somalia, however, that is potentially most troublesome. Some specialists say this could backfire on the United States.

The fear is that Somalia's mercurial leader, President Mohamed Siad Barre, will use those arms -- and the symbol of American support that they might be made to represent -- to renew the fight with neighboring Soviet-backed Ethiopia for the disputed Ogaden Desert region.

Critics argue that in any new war, Ethiopia, far larger and more heavily armed, is likely to prevail, and the U.S. aid to Somalia could have the effect of increasing internal support for the government in Ethiopia just at a time when it shows signs of breaking down.