Defence: Reagan plans largest U.S. military buildup since Vietnam in an effort to replace America's armored paunch with more punch

President Reagan has embarked upon the most massive expansion of US military might since the Vietnam war in his first 100 days as the nation's chief executive.

Appreciating the support both in Congress and the country for strengthening US defenses against an apparently inexorable Soviet military buildup, the President proposes to spend $1.5 trillion over the next five years on warships, pay raises, increased readiness, and such assorted new weaponry as the M-1 tank, the B-1 bomber, and the MX intercontinental ballistic missile.

Spending of such magnitude has no precedent in the peacetime history of the United States. In 1986 alone, the nation's defenses will devour an estimated $ 367 billion -- more than twice the sum spent on defense in 1980.

In short, the Reagan administration intends to increase the proportion of the budget devoted to defense from a quarter to a third. Some detect grave dangers in such a goal, however, pointing out that if the economy fails to boom -- a necessary prerequisite for the current rearmament program --dash all hopes of balancing the budget by 1984.

To date, the President has encountered few, if any obstacles to effecting a veritable sea-change in the nation's defense posture.

Earlier this week, the Senate Armed Services Committee approved a bill authorizing the funding of numerous weapons systems, extensive operations and maintenance activities, and various research and development projects to the tune of $134.4 billion. The administration received virtually all the cash it requested, as it seems likely to do in the future.

But if the President's first 100 days have been reasonably tranquil for the nation's defense managers, their energies consumed in planning the expansion of the nation's armed forces rather than in actually implementing the plan, an awesome array of problems awaits them. Some are more pressing and tangible than others, but all are expected to test the patience, resolve, and geniality that Defense Secretary Caspar Weinberger has displayed since taking up residence in the Pentagon.

The MX, for instance, has become a nightmare. The Air Force, which has always wanted to deploy the missile in the Great Basin area of Utah and Nevada, proposes to issue a final environmental impact statement on the projected siting areas this summer.

That the administration is by no means convinced it wants to shuttle 200 missiles around 4,600 shelters in the deserts and valleys of the US West is strongly suggested by the defense secretary's convening of a panel of "distinguished citizens" last march to conduct "an independent review of MX basing options." Secretary Weinberger has expressed interest in sea-basing the MX.

Opposition to land-basing the strategic missile is rife, and not only among Indians, ranchers, and environmentalists, who fear it will be deployed in Utah and Nevada, but among some of those more conversant with it than most -- including former CIA Director Stansfield Turner.

A considerably less agonizing decision confronts the administration in the shape of a bomber to replace the aging B-52. A Pentagon study group is reported to be on the verge of recommending construction of an updated version of the B-1 bomber -- 110 of them to be precise -- at a cost of up to $20 billion. President Carter canceled the B-1 in 1977, claiming that new air-launched cruise missiles carried aboard B-52s would stand an incomparably better chance of penetrating Soviet air defenses than the B-1. Last Wednesday, the Senate Armed Services committee approved $2.4 billion apiece for the bomber and the MX.

The Pentagon's plan to build a new Nimitz-class nuclear aircraft carrier continues to generate controversy in defense circles. "Unlike the Imperial Japanese Navy, the Soviet Navy is primarily a submarine navy," Observed Sen. Gary Hart (D) of Colorado, a member of the Senate Armed Services Committee, in a recent speech. "Can one fight a submarine navy with big carriers loaded with attack aircraft? The answer is, at best: probably not."

Among a multitude of tasks confronting it in the coming months, the Pentagon has to find additional air- and sea-lift for the Rapid Deployment Force; improve the delivery rate of both ballistic and attack submarines; and, in the view of some observers, speedily assault its much publicized problems with fraud, waste, and mismanagement that cost taxpayers billions each year.

The Pentagon is acutely aware of the need to field new strategic systems as soon as possible to counter the Soviet Union's admitted superiority. But planners appreciate that they cannot be operational for several years.

Other, less tangible but equally decisive, challenges face Reagan defense planners in the years to come. The nation's decaying industrial base has to be revived and questions of morale and fighting spirit within the armed services have to be addressed in the opinion of some.

"American officers today are no longer students of war. Rather they are students of managerial techniques," wrote Stuart Koehl and Stephen Glick in "The American Spectator" after the abortive Iran rescue mission last year. "They have lost contact with and refuse to acknowledge the nature of war, which is killing the enemy."

This view is gaining currency, it seems. Irving Kristol, professor of social thought at new York University's graduate school of business, recently noted that in the Pentagon "all the emphasis seems to be on the managementm of men and material, not on military leadership and certainly not on military pride and morale."

Many defense analysts believe that the US is developing weapons systems that are so technologically sophisticated as to be unreliable in the tumult of battle."If what we are buying will not work on the battlefield, the n it does not matter how much of it we have," declares Senator Hart.