In launching the biggest peacetime rearmament program in U.S. history, President Reagan has done for national defense what Lyndon B. Johnson did for his Great Society social programs: Reagan has virtually locked the nation into considerably higher defense spending for the foreseeable future.

Before Vietnam changed everything, Johnson persuaded Congress to start more than 60 Great Society programs for education, health, conservation, cities, consumers, the aged and the poor that soon gained a momentum of their own.

By deploring the unchecked growth of this social spending since the 1960s while the Soviet Union devoted its resources to military expansion, Reagan similarly won congressional approval during the past two years for restructuring the federal budget to give his defense buildup so much momentum it will be difficult to slow down, much less stop or reverse.

The Congressional Budget Office recently estimated that so many weapons have been ordered under Reagan that the procurement account in the Pentagon budget will jump from 26 percent of the total in 1981 to 38 percent in 1987. These weapons are the hardest to cancel.

Congress authorizes money for a weapon one year but lets the Pentagon spend it over a number of years in the future. The process is like putting money in the Pentagon bank and letting the military services draw it out as bills from defense contractors fall due.

"We are creating military entitlement programs" by ordering weapons today that will have to be paid for tomorrow, warned Sen. Carl Levin (D-Mich.), a member of the Armed Services Committee.

As a weapon moves from the drawing board to the production line of a defense plant, a political constituency is born. Management and labor and their allies in the federal government fight to keep the program alive.

As the weapon takes shape in the plant, installment payments from the Pentagon get bigger. The biggest bills for the weapons Reagan ordered and Congress approved in 1981 and 1982 will not fall due until the second half of the decade and beyond. This helps explain why Reagan's Office of Management and Budget has projected that defense spending will leap from $213.9 billion in fiscal 1983 to $364.6 billion in fiscal 1987. All told, Reagan said he expects to request $1.68 trillion for the five-year period -- fiscal 1983 through 1987 -- and spend $1.46 trillion.

Canceling major defense systems like the B1 bomber and Nimitz nuclear aircraft carriers would eventually save more than $40 billion, but it would not immediately reduce military spending or help reduce big federal budget deficits in the next year or two.

The projected spending on the two Nimitz carriers that Reagan requested and Congress authorized in the fiscal 1983 budget illustrates this "bow wave" phenomenon. A total of $6.87 billion was authorized for the two carriers. But the Navy expects to spend only $165 million of that amount in fiscal 1983 and $790 million in fiscal 1984. The really expensive work will not start until 1985, after Reagan's first term.

Another problem in canceling a weapons system is the termination costs owed a contractor when his project is scrapped. The Air Force said it had to pay $271 million in termination costs after President Carter canceled the B1 bomber in 1977.

Defense Secretary Caspar W. Weinberger argues that it would now be foolish for an increasingly skeptical Congress to slow down the president's rearmament program. "We do assign priorities, and there are some very high priorities that we can't give up," Weinberger said in an interview with The Washington Post.

"We can't give up strategic modernization" of the weapons for nuclear war, he said. "We can't delay it. We can't give up readiness of forces to fight by cutting operation and maintenance accounts . We've already got a huge operation and maintenance backlog. We certainly give up salaries; we can't go back to inadequate ones and still keep people."

"So," said Weinberger in a voice weary from making the point so many times to lawmakers looking for places to cut the defense budget, "making reductions is not easy, as many people have come to realize as they have gone through this."

Asked whether he has allowed the military services to buy too much too fast, building a bow wave of spending that will hit the government budget hard later in the decade, Weinberger replied:

"This is not a grab bag of various items picked off the wall or the shelf. This is all part of what is essential to do to maintain our ability to deter attack, both conventional and strategic, and maintain national interests in various parts of the globe. We are doing it in the most economical way through multiyear procurement," which calls for committing the Pentagon to buy a given number of weapons over several years so a contractor can lower his price through long-term planning and mass production.

Buying weapons at a slower pace to reduce the federal budget deficit would make rearming America "much more expensive and much more dangerous because you take a long time to do it," Weinberger said. "So it isn't a matter of stubbornness, and it isn't a matter of destroying the national consensus. It's a matter of trying urgently, desperately to do what the president's program and the national safety requires."

Whether Weinberger will have as much success in making that case to Congress during the next two years as he had in the last two is in doubt as Congress, returning to Washington this week for a brief lame-duck session, begins its agonizing reappraisal of how much is enough for the national defense.

Although there likely will be attempts during the session and in the new Congress next year to kill big-ticket weapons like the MX missile, B1 bomber, F18 fighter plane, Divad anti-aircraft gun and Nimitz aircraft carriers, the odds against canceling many such procurement projects are long.

More vulnerable, and frequently cut in the past, is the Pentagon budget for operating and repairing weapons the Reagan administration inherited as well as those it has bought. But there are big risks in cutting here as well.

As the new weapons ordered under the Reagan military buildup are produced and delivered, soldiers, sailors, airmen and Marines must be trained to use and repair them. Billions also must be spent on spare parts.

Cutting operation and maintenance money generates reports about divisions not ready to fight, ships that cannot sail and planes confined to hangars. Such reports helped defeat Jimmy Carter and elect Reagan.

There is a growing body of evidence in both public and secret reports that the Pentagon has not set aside enough money in Reagan's five-year defense plan to operate and repair all the weapons that have been ordered. The Congressional Budget Office estimated it will cost 36 percent more to operate the Army's new M1 tank than the M60A3 it will replace. Analysts figured the M1 would burn four gallons of fuel every mile, compared with two for the M60A3, and that mechanics will just throw away faulty components on the M1 rather than fix them.

Although Weinberger and Deputy Defense Secretary Frank C. Carlucci have vowed to keep future defense costs from running out of control, the record indicates that projected procurement and operation costs, together with congressional cuts imposed so far, have forced a retreat from some of the earlier objectives of Reagan's defense buildup.

In his secret policy guidance to the military services last year, Weinberger called for putting an additional 200,000 men and women in the Army, Navy, Air Force and Marine Corps to cover more of the world's trouble spots. He also said in his defense report to Congress this year that the Pentagon planned to increase the 24 active duty wings of the Air Force and the 12 of the Air National Guard by two each by 1986.

But Gen. E.C. Meyer, the Army chief of staff, decided to buy more hardware rather than put more soldiers on the his payroll. Even in Reagan's $1.68 trillion five-year defense spending plan, there was not enough money to do both.

Weinbeger said in an interview that the proposed additional Army divisions and Air Force wings "were not given up lightly. But there was no way to make reductions in the president's budget without some losses. I don't know if we'll go back up to 100,000 more troops."

Given the difficulties in finding acceptable programs to cut, many veteran lawmakers are predicting that, with the possible exception of the MX missile, most cost reductions forced on the Pentagon will bring changes in "force structure." This would mean slimming down or eliminating Army brigades or divisions, Navy ship crews or Air Force wings through attrition.

"This is how cuts should be absorbed," said Chairman John G. Tower (R-Tex.) of the Senate Armed Services Committee in an interview. He said nobody wants to eliminate jobs attached to weapons procurement or go back to planes that cannot fly and ships that cannot steam.

Keeping fewer sailors, soldiers, airmen and Marines on the government payroll brings immediate reductions in spending, but manpower cuts would not be enough to prevent Reagan's rearmament program from ballooning in cost later in the decade. Noting the increasing proportion of the Pentagon budget earmarked for procurement and the soaring cost of maintenance for new weapons, the Congressional Budget Office warned recently:

"These trends suggest the long-term importance of defense decisions to be made in the next few years. If the defense buildup proceeds as planned, it will be increasingly difficult to cut back substantially on defense outlays in later years without cutting the operating accounts that contribute to defense readiness."

Although members of Congress may talk during the lame-duck session about spending less for guns and more on butter, they will be so rushed to finish the fiscal 1983 military appropriations bill that the most important decisions on defense probably will be put off until next year and the next Congress.

Votes on the MX missile and Nimitz carriers will show whether the pro-defense constituency in Congress has been weakened because Reagan has tried to do too much too soon. The election results, which gave Democrats 26 more House seats, also suggest that many voters feel unemployment should be given higher priority than defense.

If Congress should move deeply into consideration of how much defense spending is necessary, a number of fundamental changes have been suggested by reformers who contend they would save billions without jeopardizing strength.

These include:

Admitting that land missiles cannot be made invulnerable and restructuring strategic forces accordingly, including canceling the MX. According to this argument, the Trident II submarine missile now being developed will be accurate enough to fulfill the MX mission of blowing up Soviet missiles in their silos. The Trident II, like the MX, can carry 10 warheads. Four Trident submarines could carry 96 of the silo-busters, four short of the number of MXs Reagan intends to deploy outside Warren Air Force Base in Wyoming.

* Slowing Army modernization. This calls for putting only a portion of the force in first-class fighting condition over the next few years rather than having money run out before any divisions are fully re-equipped. "If fiscal objectives have to be met in the short run and the flow of fast money has to be controlled," wrote William W. Kaufmann, who has written posture statements for a number of defense secretaries from both parties, "prudence may call for the preservation of 100 percent effectiveness in 30 percent of the force, and 70 percent effectiveness in the remainder, rather than an effectiveness of 80 percent across the whole force." The shortfalls could be made up when the federal deficit was not as menacing as it is today.

* Forcing the four services to consolidate aircraft and helicopters into one air force rather than continuing to allow each to have its own, with costly duplication.

* Shutting down inefficient production lines even if this means putting some defense contractors, including aircraft companies, out of business.

* Repricing the entire Reagan rearmament program, especially operation and maintenance costs, and then canceling enough procurement programs to keep future military spending within projected totals.

* Revamping the military retirement program which now enables a soldier to retire at half pay after 20 years services and three-quarters pay after 30.

* Transferring the responsibility for developing, producing and testing weapons from military officers to civilians.

Such policy changes, even if adopted by the military establishment, could evolve only slowly, leaving the more-urgent question of whether and how to cut the Reagan defense budget this year and next in response to the needs of a depressed economy and the changing political winds.

NEXT: Impact on the economy