Just as the American nuclear debate seems to be settling into dogged trench warfare, John Steinbruner of the Brookings Institution has come along with a striking and sensible analysis pointing to the way we all might go.

Not a moment too soon. Next year is bound to see a climactic political confrontation over nuclear policy. The administration is heading toward a triple showdown: 1) with the Soviet Union, at the tables where intercontinental missiles (START) and Europe-based missiles (INF) are being negotiated, 2) with the European allies, whose readiness to deploy new missiles will be tested in a tug between Moscow and Washington and 3) =with the significant sections of the American public dismayed by Ronald Reagan's nuclear policy.

There is in the administration, and no doubt in the president personally, a strong current holding that such a triple showdown is unavoidable, necessary and perhaps therapeutic. In this view, further temporizing is dangerous: candidate Reagan made no bones about his dire judgment of the nuclear balance and of Soviet intentions, and President Reagan is duty bound to act accordingly. In pursuing this policy, Reagan is being true to an important part of himself and of his core constituency.

But he is also exposing American interests to a measure of risk at once gratuitous and out of proportion to the gains -- themselves sure to be fleeting and destabilizing -- that even the full success of his policy might bring.

Hence the value of Steinbruner's new Brookings Review essay, "Fears of War, Progams for Peace" in which he takes an impressively firm grip on both the strategic and political halves of the current equation.

Strategically, he summarizes the overwhelming case that, given the nuclear arsenals of the two powers, and their catch-up capacities, nothing either one of them does from here on will make a major difference in the strategic balance. Reagan, unfortunately, still appears to believe that superiority is achievable and worth achieving.

Then Steinbruner says that, since the opportunity to prevent the creation of great destructive forces has been lost, attention must turn, and is turning, from issues of their size and capacity to managing safely the complex interactions between them. He defines the appropriate goal as a stable sense of mutual restraint, one that can get us both through the international tumult that surely lies ahead.

He concludes that, politically, the issue of restraint is ripe.

The internal American debate now pits an administration threatening Soviet negotiators with prospective force increases, against arms controllers arguing for restraint. "So far each side has been able to retard the program of the other," Steinbruner observes. "We are a house divided."

He would unite it with a "reasonable compromise" designed to bring Soviet- American agreement; Reagan's START proposal, which he, like many others, finds impractically one-sided, is defended even by its partisans as being not so much negotiable as necessary to repair an asserted imbalance.

For the Reaganites, Steinbruner suggests, there would be substantial force reductions, which could largely be done within the SALT II framework. For the freezers, deployments of old weapons would be held under the SALT ceilings and modernization of new weapons halted.

The freezers would almost certainly leap at a "reasonable compromise." The last thing most of them seem to expect is the literal success of their cause. The freeze is simply the means chosen, quite accidentally, to put pressure on the president after he had trod harshly on their nuclear nerves.

So it comes down to the president. He sets the tone and the terms of the national nuclear debate. So far his concessions to critics have been pretty painless. He has sought to identify himself more closely with arms control, and he has exchanged his early standoffishness for a seat at two bargaining tables with Moscow.

It would be much more difficult to go for a "reasonable compromise." He would have to decide that his original approach promises ill in the negotiating arena, in the alliance and perhaps in domestic politics, too. The unforeseen crisis aside, this will be the great foreign policy test of the Reagan presidency.