THE SEASON IS starting in which the annual ritual debate over defense expenditures takes place. The documents produced so far by the Reagan administration--the proposed federal budget itself and the defense secretary's report to Congress--and the immediate reaction to these documents by the critics, suggest that, in certain respects, only the numbers and the names of the players have changed. The broad lines of argument remain the same.

To the charge that the country cannot afford these gigantic expenditures, that they are both helping to ruin the economy and absorbing funds direly needed for social services, come the equally familiar replies. These are that defense expenditures as a percentage of GNP are relatively modest, that much is needed in the way of modernization and improved readiness for American forces, that social spending (if you include the big insurance programs) is still the far larger share of budget costs, that no duty of government takes priority over the duty to protect the populace from external threat and so on. All of it true--on both sides of the argument: we can't afford it, but if it is necessary, of course, we can.

From here the argument will veer off to what is meant by necessary and from there to various strategies and weapons systems and whether they are any good or not, and sooner or later there will be some congressional cuts--and after that, maybe in a few months, everyone will agree that the "wrong" cuts were made. Much sighing all around. A high- ranking officer in the Pentagon will allege that he had to counsel against going into some place or other because we have let our military resources run down so, and a complaining congressman will swear that the latest fighter-bomber plane won't fly and costs twice as much as it was supposed to. Then everyone will get ready for the debate the following year.

Don't be misled by our weariness with this debate into thinking that when it starts up we don't plan to be there, sinking into the dreadful details with everyone else concerning costs and weapons characteristics and the rest. But for now, before all that gets going, something else strikes us as far more urgent to consider. Before you can reach the question of "how much is enough," you really have to have some rudimentary idea of what it is supposed to be enough for. The Defense Department's careful descriptions of the kinds of engagements (and deterrent effects) various weapons systems and force levels are intended for and its verbal tour of the trouble spots of the world do not satisfy this need. And here, it seems to us, you come right up to the huge, troubling question concerning this country's defenses and its defense expenditures: it often seems as though the elaborate military enterprise itself is a work of fantasy, that it is absorbed in anxieties and contingencies and scenarios that have little connection with the actual world in which we live.

To some extent this is a result of interservice politics and bargaining--plans are made and weapons procured that do more for the various services' self-images than for their ability to defend. But there is more. Look at the places that, from a national security point of view, have troubled our government most in recent years--Afghanistan, Iran, Poland, El Salvador--and consider that in each, somehow the circumstances have been deemed "complicated" in ways that made irrelevant the kind of forces that we have at our disposal. This is not a suggestion that this country should have "gone into" any of those countries in a military way, only a comment on the split-screen quality of our costly national defense establishment. There is our military enterprise and then there is what we do around the world.

The fact is that however much improvement may be needed in this country's military forces, the show of strength that is relevant and required now has to do with political will on the nonmilitary diplomatic and economic front. It's all very well to talk tough about weapons and forces and their great potential. But a country that is not willing to sustain a grain embargo or other nonmilitary pressures that are controversial and inconvenient at home, really isn't going to impress anyone with lots of added hardware. You can agree or disagree with the substance of Secretary Caspar Weinberger's blast, in his report to Congress, against current trading arrangements with the Soviet Union. Maybe it wasn't even his to have brought up in such a report. But, right or wrong, there is a certain relevance to it: the real questions concerning this country's strength in its conflicts around the world are only partly questions of force levels and organization. They are, in much larger part just now, questions of national purpose and credibility on a variety of nonviolent fronts.