The Reagan administration is jeopardizing its record peacetime military buildup over the long run because it has failed to persuade people that such enormous sums are necessary, are being spent wisely or fit into a coherent strategy for defending America's interests.

That is the general theme that emerges from interviews with some of the most experienced defense specialists outside the Reagan administration, including three former defense secretaries, when they look at how defense will fare over the long run.

These former officials and congressional experts are all supporters of a strong national defense and had some good words for President Reagan's efforts the past two years to shore up quickly some obvious weak points in military readiness.

They are not predicting big cuts in the lame-duck session of Congress, but they believe such cuts are inevitable because of the way the administration has managed the buildup thus far.

They also express serious doubts about how much more secure Reagan's planned $1.6 trillion, five-year military spending plan would leave the country.

One of the former defense secretaries, longtime Republican strategist Melvin R. Laird, has become so concerned that he drafted a memo to the president about the possibility that Reagan's defense spending strategy may boomerang.

These key points emerged from the interviews:

* The administration, which came into office with what one observer called a "golden opportunity" to build up the country's defenses over the long term, is in the process of losing the public support it once had. Its proposed spending increases are far too steep and it has not been able to demonstrate a need for them.

* A break in the present pro-defense consensus in the country could ultimately cause sharp reductions in defense spending and actually leave the military worse off than it otherwise would have been in some areas years from now. With so many new weapons programs under development, there will be even fewer ways to cut the defense budget without knifing into the manpower, ammunition, training programs and spare parts needed to react quickly to an emergency.

* The administration's adoption of a global strategy in which American forces could be expected to fight on several continents and oceans at once has produced a lack of discipline and realism in defense planning and therefore a lack of priorities. That strategy is so broad that not even the $1.6 trillion Reagan has asked for is enough to carry it out, military and civilian experts said.

* The feeling is widespread that the administration has allowed every major weapons project on the books, new and old, to go forward, cutting nothing of consequence, giving the military a free hand and throwing money at the defense problem in part to send a signal of toughness to Moscow. Experienced officials question whether the country needs and can afford two new bombers instead of one, whether there is a need for a huge expansion of the U.S. fleet to more than 600 vessels, and whether the Army can manage wisely some 300 new projects simultaneously.

* American foreign policy also has a great impact on overall national security. The concern here is that the administration has produced strains in U.S. relations with China and Russia at the same time. Serious disputes with allies in Europe have erupted over economic issues. And the United States, critics asserted, has not used the leverage of its huge defense buildup to force allies in NATO and Japan to step up their defense outlays too. The point is that if war comes, the United States cannot win alone.

* There is considerable disappointment, usually uttered in private, with Caspar W. Weinberger, the likable and intelligent secretary of defense who many specialists feel has failed to get a grip on the Pentagon and spends too much time traveling abroad. The disappointment stems from Weinberger's earlier reputation as a tough-minded analyst and budget-pruner in President Nixon's Office of Management and Budget.

Weinberger rejects all of this.

The five-year defense plan, he said in an interview, "is not a grab bag of items picked off the wall." Rather, he said, it is "a single, cohesive, coordinated program designed to restore deterrence" of either conventional or strategic nuclear war.

"When people challenge the strategy we are using and the means we are using to catch up I always ask what is it that we should give up. Should we give up Korea, Japan, the oil fields, Europe, the Caribbean, the protection of the United States? So, no one has suggested what we should give up," Weinberger said, while in his view the Soviet threat continues to expand "very substantially."

Stressing a massive Soviet defense buildup in recent years and consistently larger Soviet defense budgets, Weinberger said: "When you look at a map of available Soviet air and naval and access to materials and facilities now compared with 10 or 15 years ago . . . all you see are red blotches . . . it looks like the whole place has caught the measles."

The United States had its period of scaling back defense "for 10 years at least while the Soviets did not," he continued, adding that the "people emphatically rejected in 1980" the Carter administration's approach. "So we are trying to recover enough strength in time, and nobody knows how much time we have," he said.

From the outside, however, things look different. Many experts see Weinberger and his huge budget heading for sharp cuts that will put defense on a roller-coaster. It will lead ultimately, they feel, to a weaker defense than could have been achieved with more modest annual increases that could have been sustained over a longer period.

"We'll end up with a helluva big cut, a tremendous cut," said former Nixon administration defense secretary Laird in an interview, "because you are not building the popular consensus you need now. The peaks and valleys for the Defense Department budget are the worst things that can happen as far as the long-term national security of this country."

Laird, a former eight-term congressman, argues that the country is at a critical juncture and that Reagan must decide how to exercise leadership to conserve the defense consensus. Laird warns that there has been a blanket go-ahead for new and old, bad and good, weapons rather than a necessary weeding-out.

He also warns of grave miscalculations about the eventual costs of the announced military strategy and is urging the administration to make a major, one-time correction, a scaling-down of defense spending from a planned average through 1987 of 8.1 percent of gross national product to 6.5 percent that would save some $68 billion.

"The defense consensus has already been destroyed," added James R. Schlesinger, a defense chief under presidents Nixon and Ford. "I think we have probably built ourselves into a position in which we are going to erode our readiness and sustainability of combat forces somewhere starting around fiscal year 1985," he said. That is when all the bills come due for big hardware projects now under way and there will not be enough money or public support to pay for them, he said.

"We will get a curtailment of resources that is probably even more unwise than the initial excess of resources," Schlesinger added.

Harold Brown, President Carter's defense secretary, said of the Reagan defense program that "the money's not enough for the projected program and the projected program is not enough for the projected political commitments" around the world. While that is something of a traditional failure of many administrations, Brown said "I think it's worse this time, partly because I think there has been less attention to priorities. That's understandable in a way because when you foresee the addition of large amounts of money then the pressures for priorities at least temporarily weaken."

"We are not going to be able to do, with the $1.6 trillion, what is planned," said Gen. David C. Jones, the chairman of the Joint Chiefs of Staff who retired in June. "So therefore there is a murky future about what will really come to fruition."

"But my big concern," Jones added, "is that we are getting more disjointed in the sense of not having fully coherent programs or capabilities. Admiral Gorshkov, the head of the Soviet navy, once said that the job of senior people in defense is to try to make the total capability greater than the sum of individual parts. The way we have developed over many years, in the Pentagon and in Congress, we have just the opposite. Our total capability is less than the sum of the individual parts."

The mismatch, Jones said, between strategy and the forces to carry it out "is greater now than it was before because we are trying to do everything."

Sen. Sam Nunn (D-Ga.), a member of the Armed Services Committee and a respected commentator on military affairs, said there is no doubt that some improvements will come out of the Reagan program. "But the real question is whether we are going to improve as much as we should with the amount of resources going into it.

"My answer to that is that we will not unless there is a clearer sense of direction as to what our national security strategy is. I don't think this administration has yet clearly articulated that. Weinberger has said several times that 1 1/2 wars or 2 1/2" -- some prior administrations cast their defense planning in these terms, saying that was how many wars they were preparing to fight--"are not sufficient or proper labels. If that's the case, then what are they building for. That's what I'm still puzzled about," Nunn said.

The Army chief of staff, Gen. Edward C. Meyer, answers Nunn's question this way. What the Reagan administration is trying to do, in conjunction with allies around the world, is "go back to essentially a containment policy . . . and that means being able to respond rapidly to challenges anywhere around that periphery and anywhere else where the Soviets have been able to expand and project power.

"There is no clear 1 1/2 to 2 1/2 war matrix. There is a clear one war but the rest are all hazy and I think Cap Weinberger says it correctly. He says that if you are to rule out any area of the world then you provide a place for the Soviets or Cubans to go stirring around," Meyer said.

But Rep. Les Aspin (D-Wis.), a member of the House Armed Services Committee and former Pentagon official, does not think that Weinberger has been laying the issues out correctly. He evaluates Weinberger's performance as "not very good. I can understand his philosophy to spend more . . . and some of that, for pay and readiness and spare parts . . . is good.

"But what's hard to understand is that he seems to have failed to get a grip on the operation; [there] is a strange lack of skepticism about military weapons for a guy who has been through OMB and a real lack of putting forth a philosophy of what we are trying to do with defense," Aspin said.

"You see you don't need that when people want increased defense spending. When you need it is when the increase comes under attack and there has not been a plan or a rationale put forward to get us from here to there along this path," he said.

Weinberger's annual report to Congress last March, Aspin said, "was the worst damn posture statement I've ever seen. You went right from 200 years of Russian history into the need for five new tanker aircraft . . . without any kind of rationale. This leaves the whole system open to the charge of throwing money at defense in the same way that Republicans used to criticize Democrats for doing it in social programs."

To a great extent, it is the $96 billion, five-year expansion of the U.S. Navy from its current 481 ships to more than 600 vessels by the late 1980s that mirrors the larger debate over the overall defense program. The naval expansion, proposed and defended by the most articulate and aggressive of the civilian service chiefs, Navy Secretary John F. Lehman Jr., symbolizes the Reagan strategy of being able to fight in force all over the world.

There is little quarrel by the outsiders with the idea of some expansion and modernization. It is the dimensions, cost, mission and impact on other services that are in dispute.

Army leaders are known to fear that the cost of the Navy will eventually come out of the Army's hide.

Lehman has said the U.S. Navy needs to deal "simultaneously with conflicts in the Far East, the Near East, the Persian Gulf, the Indian Ocean, the oil lifelines around Africa to the United States and Europe, the Mediterranean, the North Atlantic, the Caribbean and the eastern Pacific."

"The Navy," Laird said, "is going wild by making all these commitments on ships. It hasn't been proven to me," he added, "that you need a Navy that large. I don't think we can afford it," suggesting, as do others, that a fleet of perhaps 550 vessels is enough.

Although Lehman also puts some emphasis on the more traditional Navy role controlling the sea lanes, Schlesinger, Aspin and others would put more emphasis on this role and especially the building of more attack submarines to guard convoys, bottle up Soviet outlets to the sea, and counter Soviet subs. The Pentagon could buy a lot of extra submarines, Aspin argues, for the $32 billion it will cost to add two more nuclear aircraft carrier task forces to the existing 13 carrier battle groups, as Lehman wants.

What concerns Nunn the most is that "Lehman keeps talking about using the carriers to more or less take the battle to the Soviets" close to the Soviet land mass. "I just think that is an unrealistic use of carriers based on my conversations with many Navy admirals," Nunn said. "The last thing they want to do is bring those carriers into a confrontation within reach of Soviet air power based on land. That's too big an asset."

Another major challenge to the Reagan program from the specialists is the decision to build both the B1 bomber and then the more futuristic Stealth bomber.

Laird, Brown, Nunn, Aspin and others would drop the B1 and concentrate on Stealth. Although there is some danger in this, Nunn said buying only 100 B1s as is now planned is neither economically nor militarily sound. Furthermore, once Stealth is in service, the B1 role will be as a cruise missile carrier rather than as a penetrator of Soviet airspace "yet an enormous percentage of its cost is for the penetrating role" of getting through thick Soviet air defenses, Nunn said.

There is less dispute over the new MX land-based intercontinental missile, although here, too, Laird and Nunn believe the nation ought to begin moving away from land-based missiles and toward more reliance on the submarine-based missile force which, unlike the land-based, remains basically invulnerable to a Soviet attack.

Last March, President Reagan declared that "on balance, the Soviet Union does have a definite margin of superiority" in strategic nuclear striking power. This was a reference primarily to larger and more numerous Soviet land-based missiles in comparison to a U.S. edge in submarine-based missile power and bombers. Because land-based missiles are most accurate, however, they are most threatening and are the key force driving the arms race.

Reagan's remark on inferiority is disputed by many specialists. But it has locked the administration into putting major new emphasis on a strategic buildup that ranges, because of what critics say is a lack of priorities, from worthwhile projects such as better control over nuclear forces to questionable efforts on civil defense.

Gen. Jones said, "I have never used the word inferiority. That is too absolute in the implication that some Soviet advantages can be meaningful. They can be meaningful only if we allow them to be so . . . if we feel inferior . . . if we feel in a crisis that they could intimidate us.

"I have no doubt that we can deter the Soviets," he continued. "You can hold all kind of war games that show some difference in outcome but in any calculation there is unprecedented devastation on both sides."

Jones, a supporter of MX, emphasized that "certainly we need to do some modernization of our strategic forces, but I'm not one to believe we've got a lot of catching up to do."

While there is no question among the defense experts outside the Pentagon as to the extent of Soviet power, and there is concern over local balances of power, there is a different view of the relative strengths of the two superpowers than that presented by the Pentagon.

Jones, Schlesinger, Brown and others say U.S. air and naval forces remain superior to their Soviet counterparts, although the Soviets have an easier mission in many cases. If anything, the U.S. naval buildup will increase an existing U.S. advantage, Brown argues.

The general readiness of Soviet forces and Moscow's ability to keep them supplied is also overrated in the West, other officials claim. Brown also makes the point that allied navies in NATO are far better than Warsaw Pact navies.

Gen. Meyer, only half jokingly, said the question of whether China and Russia have a reconciliation "is more important than 15 divisions, three carriers or 100 MX missiles in many ways." If the communist powers do make up, then vast numbers of Soviet troops on the Chinese border could be redeployed toward the west. Meyer stresses the importance of skilled foreign policy in helping make defense efficient.

Most of those interviewed argued that nuclear war is the least plausible threat to the United States and that conventional forces -- troops, planes, ships plus air and sea transports to carry troops abroad -- are most important now in maintaining deterrence.

Yet it is precisely these unglamorous items in the defense budget -- Army troops levels, the quality and readiness of U.S. troops, the airlift and sealift needed to transport them -- which yield the quickest savings when either Congress or the Pentagon is forced to make cuts. So oddly enough it may be here, rather than in the more esoteric and visible world of new weapons, that the real battle for future preparedness will be fought.