Because Budget Office civil servants are once again insisting on meat-ax cuts in military spending. President Carter is finally being compelled to resolve his agonizing ambivalence on the defense budget.

Career defense specialists in the Office of Management and Budget have directed the three services to prepare budgets reducing their spending in the next fiscal year by 9 per cent. What's more they are pressing for some specific reductions approaching that figure requiring cuts in aircraft carriers and aircraft.

That is an entirely familiar OMB refrain. What is new are responsive vibrations from key staffers at the National Security Council and doubts that the Defense Department's new civilian leaders will fight hard enough against the cuts. The biggest question of all: How truly committed is the President to the 3 per cent annual real growth in defense spending he promised to his NATO partners last spring?

OMB's long-standing determination to save money by slashing away at defense has triggered wide-rancing troubles in the past. Its major architect is one of those anonymous civil servants who wield enormous influence in Washington: Richard Stubbing a 15-year Budget Office veteran who has been deputy chief of the National Security Division since 1974.

Stubbing is credited by the Pentagon with talking then-OMB Director James Lynn into proposing huge defense cuts in 1973 - setting off a fateful chain of events. James Schlesinger then Secretary of Defense waged so heated a campaign against the reductions that President Ford sacked him. The amounts cut were later restored but the Schlesinger affair encouraged Ronald Reagan's challenge and undercut Ford's presidential campaign.

With new, green civilians coming to OMB this year. Stubbing & Co were pedding the same old wares. In particular, a cost-cutting scheme to reduce the Navy's aircraft carriers from 12 to eight bears the Stubbing trademark.

Papers sent to the Pentagon by OMB two weeks ago are cloaked in bureaucratic ambiguity essential for self-protection. Besides being asked to prepare the 9 per cent cut ("decrement budget"), each service is also directed to prepare a 3 per cent increase. But there is no doubt OMB wants cuts. OMB Acting Director James T. McIntyre is talking about a ceiling some $5 billion below what the Pentagon feels it must have.

But how does the square with the President's promised 3 per cent annual increase increase? The ingenious OMB rationale is that Carter did not mean what he said but intended to limit the increase to NATO-related defense spending (which would mean no more than a 1 percent overall increase).

That opens up all manner of word games. For instance OMB staffers want to knock out funds for the AMST jet-powered cargo plane proposed by the Air Force. Although invaluable for Western Europe, it is not included in the NATO-related 3 per cent increase as defined at OMB.

Ordinarily, this sort of shell game would be rejected out of hand by NSC staffers, particularly with U.S. resolve in doubt around the world. But many of the new breed of foreign-policy specialists brough in under Carter are wholly sympathetic to the idea of unilateral defense cuts. Thus, remarkably enough, there is divided opinion within the NSC about OMB's sophistry on the 3 per cent mandate.

At the Defense Department, nobody doubts that Secretary Harold Brown is convinced the 3 per cent promise refers to all defense spending. But when a memorandum attacking OMB's meatax cuts was prepared for Brown's signature to be submitted to OMB, the memo disappeared in the Secretary's office. That is explained on grounds that the time is not ripe for such memos, since the OMB position has not yet hardened. But worried officers see this as an omen that Brown will be only a profroma advocate of what they feel is vital to the nation's security.

Accordingly, this major decision is now being thrust into the lap of the president, who finally must come to grips with an issue he has always ducked. White promising defense cuts of $5 billion to $7 billion during the campaign, after his election he promised defense increases. When NATO members started worrying about U.S. constancy, Carter hardened his views into the 3 per cent promise, later ratified in a formal presidential directive.

Although officials claim the President has not yet confronted this decision, the fact is he has spent unprecedented hours pouring over the budget. The time for choice is at hand. If the backs the OMB bureaucrats, he will win loud applause within his administration and from liberal Democrats. But the effect on other nations, both friendly and hostile, will be profound.