Largely overlooked in the mounting congressional debate over the Reagan administration's proposed five-year defense plan is the new military strategy ostensibly justifying the biggest peacetime defense program in American history. The strategy, enunciated mainly by Secretary of Defense Caspar Weinberger and Navy Secretary John Lehman, envisages a protracted, non-nuclear war against the Soviet Union waged around the globe.

As Weinberger summed it up in a recent report to Congress, "Our long-term goal is to be able to meet the demands of a worldwide war, including concurrent reinforcement of Europe, deployment to Southwest Asia [the Persian Gulf], and support in other potential areas of conflict." Lehman talks of a Navy able to "prevail" at one and the same time over "the combined threat of our adversaries" in the Pacific, Indian, and Atlantic oceans, including the Norwegian Sea, a lair of Soviet subs and in easy reach of massive Russian air power based on the Kola Peninsula.

Weinberger and Lehman reject the present "11/2- war" strategy, promulgated by the Nixon administration and calling for forces sufficient to defeat a Soviet invasion of Europe and a lesser enemy elsewhere. They seek to replace it by a giant "one-war" strategy, to be conducted against the Soviet Union on a series of widely separated fronts on and along the vast Eurasian land mass. The Soviet Union is to be deliberately and concurrently engaged not only in Europe but also in the Persian Gulf, Northwest Asia, and on the high seas. Also taken under fire will be Soviet allies, such as Cuba and Libya, that try to assist Moscow by blocking the flow of reinforcements and supplies from the United States to fronts overseas.

The Weinberger-Lehman strategy is a tall and dangerous order. Aside from the question of whether a worldwide conflict with the Soviet Union could long escape the play of nuclear weapons, the strategy is, in truth, not a one-war strategy but a multiple-war strategy, albeit against a single major enemy. It is admittedly unwise to assume or hope, as have U.S. force planners in the past, that a shooting war with the Russians could somehow be restricted to only one area of the world. A conflict that began in one area likely would expand quickly to others. The probability of "horizontal escalation" (to use the term now in vogue in the Pentagon) cannot be ignored.

What is disturbing is the relish with which horizontal escalation is being greeted by the new crowd in the Pentagon. It appears to be an article of faith that geographical expansion would benefit the United States rather than the Soviet Union. The building bubbles with talk of "military opportunities" afforded by horizontal escalation; of the "freedom" offered by a worldwide war "to assume the offensive in areas where Soviet forces are weak" and to "settle accounts" with troublesome but exposed Soviet client states.

In fact, it is Moscow and not Washington that would reap the benefits of horizontal escalation. Sitting in the center of the Eurasian land mass and already possessing preponderant military power directly opposite both NATO and the Persian Gulf, the Soviet Union could move forces from one front to another far more quickly than could the United States, separated from Eurasia by thousands of miles of water. Indeed, as the presumed aggressor, Moscow could initiate a conflict by feinting in one area, thereby diverting U.S. forces away from the intended place of decision.

Against the Soviet Union, deliberate horizontal escalation of a war is a recipe for defeat. It violates the fundamental military axiom of concentration by dispersing limited forces in the face of a larger and more compact adversary. To attempt to fight everywhere is to end up losing in most places, if not everywhere. To be sure, the United States could be expected to prevail in areas closer to home. The emotional rewards of bashing Cuba and stomping Sandinistas in Central America, however, would be paltry compensation for the loss of Europe or the Persian Gulf.

If the Weinberger-Lehman strategy is at odds with sound military thinking, it is also palpably unsustainable without enormous increases in active- duty U.S. force levels. Such increases, which for the Army alone would entail creation of at least four new divisions to permit that service to meet its Persian Gulf and NATO obligations concurrently, would require a return to conscription. Yet neither a larger Army nor abolition of the all-volunteer force is being contemplated by the administration.

The new strategy is grossly at odds even with those increases in military power proposed in the administration's five-year defense plan, which calls for increases in defense spending after inflation averaging 7.5 percent per year. Those increases, while welcome and long overdue, fall far short of satisfying the ambitious military objectives of the new strategy. The three carrier battle groups to be added to the present 12-group fleet and the expansion of land-based tactical air power from 36 to 40 wing equivalents are but drops in the bucket of requirements for a prolonged, multi-front war with the Soviet Union.

It is in any event highly doubtful whether the all-volunteer force can provide people sufficient in quantity and quality to man an expanded Navy and Air Force. The AVF is barely capable of manning the present force structure, and severe shortfalls in critical skills persist. The recent upsurge in recruiting and retention rates is attributable as much to recession-induced high unemployment as it is to increases in pay and benefits. And the worst is yet to come, given the certain relative and absolute decline of the military-aged cohort within the American population well into the 1990s.

Equally doubtful is the fiscal feasibility of the Reagan defense program, especially against a backdrop of record-breaking federal deficits, mind-boggling Pentagon cost overruns, and mounting political opposition to financing defense budget increases at the expense of domestic social and economic welfare programs.

In short, barring a return to conscription and a comprehensive restructuring of the American economy for the purposes of war, the Weinberger-Lehman strategy will serve simply to widen a longstanding gap between U.S. military commitments and capabilities. Whatever the wisdom of a multi-front, worldwide war strategy against the Soviet Union, it cannot be had on the economic and social cheap, especially against an adversary possessing a mass conscript army and willing to devote to defense 12- 15 percent of its gross national product. The so- called "half-war" in Vietnam, waged against a small, third-rate power, consumed vastly greater American military resources than those the Reagan administration's wishes to create.

The abyss separating the administration's strategy and resources was candidly conceded by Undersecretary of Defense Fred Ikl,e in February. In a public statement before the Senate Armed Services Committee that the Pentagon sought retroactively to classify, Ikle admitted that "Even an increase in U.S. military investments as high as 14 percent per year (in real terms), continued throughout the decade, would not close the gap in accumulated military assets between the United States and the Soviet Union until the early 1990s." Ikle went on to conclude that "That is a bleak outlook, implying . . . a need for a defense increase considerably steeper than what the administration now proposes."

Seemingly lost on this administration is a recognition that strategy is not just a mirror of military desire. It is, like politics, the art of the possible. Strategy involves choices within the framework of finite resources, and an ability to distinguish not only between what is desirable and what is possible, but also between what is essential and what is expendable.