President Reagan's unexpected decision to dismantle a missile submarine to keep the United States in compliance with the SALT II agreement gives the Soviet Union one last chance to try to push Reagan into a new and meaningful arms control agreement.

In effect, Reagan has invited the Soviet Union to make him him an offer he won't be able to refuse. The Soviets have never shown a talent for taking the initiative in the past, but Reagan has opened a door that at least some Soviet officials should be tempted to rush through.

In recent weeks there has been mounting evidence that the Soviets had given up hope of negotiating a serious agreement with the Reagan administration. A succession of pessimistic statements from Soviet leaders has suggested that they were on the verge of deciding that the existing web of arms control agreements was about to unravel, bringing on a new round of the arms race.

Now Reagan has sent an unexpected but unmistakably clear signal to Moscow: despite the advice of his defense secretary to abandon the unratified SALT II treaty's limits on nuclear arms, despite a clever "gray-area" proposal to dry-dock a submarine later this year instead of dismantling it as SALT II would require, Reagan opted for full compliance with the treaty. He proposes to dismantle a working Poseidon submarine and its 16 missiles later this year, when a new Trident sub will go into service and put the United States over the treaty limits for multiple-warhead missiles.

So Reagan has refused to fulfill the Soviets' worst-case expectations. To the dismay of many of his staunchest supporters, the president has opted for arms control over the hard line he rode into the White House. To keep alive the dream of an arms control agreement -- and of a summit meeting with Mikhail Gorbachev -- Reagan has taken to his bosom a treaty he has repeatedly called "fatally flawed."

It is quite a performance, one that will likely baffle many in Moscow. Some there will probably argue that this is a ruse, designed primarily to allow Reagan to continue developing new missile defenses that seem to terrify the Soviets. Other Soviets will probably question Reagan's sincerity.

But thanks to Reagan's surprise gesture, the Soviets now have an unprecedented opportunity. They could, for example, come forward with a sweeping proposal for reductions of offensive arms. Gorbachev has already spoken of 25 percent cuts, but now he could offer even more -- reductions, say, of each side's warheads by one-third, with steadily deeper reductions for a number of years afterward.

An offer like that would be difficult for the United States to refuse, even if Moscow accompanied it with stiff conditions limiting the development and testing of the Strategic Defense Initiative, or "Star Wars," systems. Could Reagan resist the pressure from the European allies and Republican senators nervously eyeing the 1986 elections to accept massive arms reductions offered by the Soviet Union?

Neither the Europeans nor the Senate share Reagan's enthusiasm for the dream of a leak-proof missile defense. Marshal Sergei Akhromeyev, chief of the Soviet general staff, indicated in an authoritative statement last week that the Soviets would accept continued U.S. research on Star Wars -- Moscow seems to realize that research cannot be banned. So Reagan might be able to leave office with deep cuts and ongoing Star Wars research -- not a bad combination to take to one's encounter with the history books.

But this is all hypothetical. To become reality, the Soviet Union would have to make a radical proposal. Given its reflexive caution and the new leadership's preoccupation with domestic problems, this may not be possible. In past arms control negotiations, the Soviets have always reacted rather than initiated proposals.

And if they don't take this opportunity, it is the arms controllers in this country and Western Europe who will then be on the defensive. The Soviets had their chance and didn't take it, the hard-liners are likely to say -- proof that Moscow never was interested in real arms control.