President Carter used the device of implicit choice to move from a discussion of Russian troops in Cuba to an appeal for Senate ratification of the arms-control treaty with Russia. This was the key paragraph in his speech to the nation on Monday night:

"My fellow Americans, the greatest danger to American security tonight is certainly not the 2,000 or 3,000 Soviet troops in Cuba. The greatest danger to all the nations of the world -- including the United States and the Soviet Union -- is the breakdown of a common effort to preserve the peace and the ultimate threat of nuclear war."

Implicit in those words is an either/or decision. Either the United States forces a showdown with Russia over the troops in Cuba. Or it moves forward toward peace and security with the Strategic Arms Limitation Treaty.

If that were indeed the choice, almost all of us -- and certainly this columnist -- would be for instant ratification of SALT. But it is a phony choice, with respect to both the troops and SALT.

As regards the troops, the president was almost insouciant in minimizing their importance. He wasn't sure whether the number was 2,000 or 3,000. Nor how long they had been there. Nor whether they were there for combat purposes, as U.S. intelligence showed, or for training purposes, as the Russians maintained.

In fact, the Russian troop presence in Cuba is an element of a global strategy the United States has found difficult to handle. The strategy uses Cubans as a proxy for Russians in the Third World. To that end the Cubans have backed up regimes favorable to the Soviet Union, and hostile to the United States, in Angola, Ethiopia and South Yemen.

In all honesty, the president's speech should have mentioned the Soviet-Cuban cooperation around the world. It should have indicated this country's particular concern with the Cuban presence in the approaches to the Middle East and Persian Gulf. It should have expressed America's determination to call the Russians and Cubans to account for their threat to this country's vital interest in that area.

With respect to SALT, the president overstated its benefits enormously. The treaty does not put a cap on the arms race. On the contrary, the immediate consequence of ratification will be an upsurge in defense spending.

Neither does the treaty express "a common effort to preserve the peace." On the contrary, the Russians and their Cuban allies have, in the very midst of the SALT negotiations, taken steps that jeopardize the peace, especially in the Middle East.

Together, trivialization of the troop issue and inflation of SALT benefits create a reverse linkage. Willy-nilly, whether he wants it or not, the president has in effect told the world that SALT is too precious to justify an assertion of other American interests.

The Russians must now be under the impression that President Carter needs SALT so badly that it doesn't matter how recklessly they behave in areas of confrontation. Worse still, the same estimate is probably being made by countries that base their actions on assessments of the balance between Washington and Moscow. The list starts with China and includes Saudi Arabia and the other oil-producing countries of the Persian Gulf.

Perhaps it was too much to imagine that the Carter administration could have used the uproar over the troops to forward this country's interest with Russia. An administration inept enough to stumble into the troop issue was unlikely to extricate itself with advantages. Indeed, realists probably ought to express relief that Carter did not make matters worse. Some of his counselors, after all, advised that he admit in public that he had made a mistake by allowing the troop issue to become so big. Others counseled that he play the China card.

But if the administration might have done worse, the Senate can still do better. Now that the president has established reverse linkage, now that he has fed the impression that the United States is loath to assert its interest for fear of jeopardizing the treaty, discriminating senators will want to right that balance. They can and should insist that, before ratification, they have from the president certification of more reasonable behavior by Russians and Cubans in areas that constitute -- far more than the threat of nuclear holocaust the president tosses about so loosely -- a true danger to international security.