DEFENSE SECRETARY Frank Carlucci has done what his predecessor always resisted. He has told the services to make their budgets for next fiscal year realistic. You can argue that Mr. Carlucci had no choice, and it remains to be seen what growth rates the budget will project beyond next year. But that misses the significance of what has happened. The defense buildup is over; the issue now is the tilt of the plateau.

The secretary has ordered that next year's defense budget conform to the terms of last month's budget summit. From a certain distance that is unremarkable. The president himself signed off on the summit. Spending authority and actual spending would both still increase about $8 billion, not enough to cover likely inflation but not exactly gruel, either. At $300 billion for the year the country would hardly go unprotected.

But a standstill budget is a cold shower for the Pentagon. When the president sent up his fiscal 1988 budget last January, he and the services were pointing toward a $332 billion military program by fiscal 1989. That is the basis on which the services were being told to plan. That target was already down $31 billion from where it had been the year before. Now Mr. Carlucci intends to reduce it 10 percent, or $33 billion, more.

Former secretary Caspar Weinberger fought such accommodative budgeting. The result was that he sent up what, in fiscal and political if not also in military terms, were frothy budgets. Congress had to cut them even though, institutionally, it is less capable of doing so well than the executive. Its instinct was to nickel-and-dime everywhere rather than cut whole programs. The services, undisciplined by the secretary, sowed the seeds for more programs than the country will be able to afford. Congress tried to slow the programs down rather than choose among them. So there is still a major squeeze ahead, which the new orders will only partly resolve.

Mr. Weinberger always balked at helping Congress cut the budget, partly on the theory that it would only gulp down what he gave and bang its spoon for more, partly to clarify who was responsible. The posture only worked for a while; in the end the secretary took himself out of the game.

A slimmer budget may put more pressure on Congress -- the Democrats especially in this election year. Mr. Weinberger ultimately made it easy for them; they could simply be for less. A realistic budget may force them to face reality in turn. The Pentagon needs to be put on a steady path; this is a step in that directio