THIS IS ONE of those truly interesting times in Washington when the policy and political context of a transcendent issue -- defense -- is under review. No longer is defense being treated chiefly as a response to international threat. It is also being forced to justify its claim, against strong civilian counterclaim, for short budget dollars. Congress has accepted a broad requirement for slowing down the administration's military buildup. The administration itself has accepted only a tactical requirement to slow down the buildup for one year. In the difference lies much grinding political conflict to come. But that's all right: there is little worth arguing about more.

Despite the truly disturbing disorder of the Reagan buildup, there is no denying the benefits it bought in power and confidence. At some point, however, there was bound to be an application of the brakes. President Reagan had raised defense spending to a level double that of 1979. He had also made it possible for others to press the questions now coming to the fore. How should defense and economic considerations be meshed? How can the United States ensure it has the defense it needs?

Currently the first question is in the spotlight. With one eye on the budget deficits and the other on a range of procurement scandals and Pentagon management maneuvers, Congress is deciding at what rate defense should grow. (Even if there is a freeze and no inflation increase, the money in the pipeline will keep spending rising through the '80s.) The issue has come down to a fairly narrow one. The Democratic House and the Republican Senate are divided over whether the Pentagon's inflation adjustment should be eliminated -- the House thinks it should be, the Senate that it should not. The Senate bill looked to us like a pretty sound and hefty cut, a sufficient one. But this presumably will now be negotiated. We think the administration has a fair point when it says that others, friend and foe, will draw weighty conclusions from the spectacle of a too-precipitate American retrenchment. But it is also true that they will draw weighty conclusions from the spectacle of America's settling in for the long haul at a level at once relatively high and, most important, politically sustainable. In either event, President Reagan has lost the control on the defense issue that he enjoyed throughout his first term. This argument is going to be resolved in and by Congress.

That leaves open the question of how the United States gets the defense it needs. Unfortunately, few people are looking at it. The administration conducted its buildup without ever showing it had fully thought out a plan for either the use of resources or the accumulation and application of forces. Its policy came down to: more. What justification there was for this approach in the earlier phase eroded as the years went on.

As it happens, Congress is much better equipped to think in terms of budget numbers than in terms of military missions to be performed and forces to be designed and produced and deployed to serve those missions. A few legislators -- among them Sam Nunn and Les Aspin -- are of a mind and a competence to think about defense as defense, and not simply as a budget-cutting exercise. They have to draw Congress into a dialogue with those in the administration who have a taste for rigorous defense planning. Even at the new peak which may be becoming the Pentagon's budgetary plateau, that is the urgent need.