AFTER A DARK spring, the arms control horizon is brightening. President Reagan, in May, had ruled that the United States would no longer be bound by terms of the SALT II treaty. The fainthearted feared this would put the kibosh on the Geneva talks. In fact, it merely fed into political currents already flowing in both capitals. The Soviets, declining to accept this rebuff as his last word, kept right on unfolding their negotiating position. This encouraged arms control advocates in the administration and Congress, who redoubled efforts to get Mr. Reagan back on their negotiating track. The latest reports hint at some success. Mr. Reagan is now said to be prepared to resume what will inevitably be a long, hard climb to a possible agreement with Moscow.

The deal coming into view would involve deep cuts in offensive arms and agreed restraints on the development and deployment of defensive arms. Sound familiar? This is the deal that became possible from the moment in 1983 when President Reagan unveiled his plan for a missile defense in space. Both supporters and opponents of his Strategic Defense Initiative, or Star Wars, could see that it deeply alarmed the Soviets -- for its promise as a vehicle of American technological and economic challenge if not for its theoretical threat as a weapon of the future. For arms controllers, the point of the exercise became to exploit those Soviet apprehensions in order to trade off SDI against what American strategists have always agreed to be the prime Soviet strategic threat, Moscow's great store of land-based missiles with at least a hypothetical capability of a first strike against the United States.

Actually, getting something of value for SDI from the Russians has been only half the battle. The other half has been to induce Ronald Reagan and the powerful Pentagon civilian partisans of SDI to accept the idea of some sort of trade. For they believe in SDI, if not as a weapon then as an instrument of challenge to Moscow. There is no guarantee now that this half of the battle has been won, or will stay won, although Defense Secretary Caspar Weinberger's remarkable public lamentations are certainly indicative. Mr. Reagan has had great difficulty holding to a constant position.

The Soviets, however, appear to be offering a formula to allow continued research, the halting of which could not be convincingly verified in any event, but to bar deployment for some period of years. This would put off a decision on SDI to a different administration and a different set of circumstances. Meanwhile, in the cooling of the passions surrounding strategic defense, the two sides could work on cutting offensive arms. There are other provisions, but this is the heart of it. Nobody can know now whether a suitable and safe agreement can be reached, but it is certainly something worth negotiating hard for.