THE talks held so far by Soviet and American diplomats have left the impression that the two sides are aiming to resolve their disagreement over Soviet troops in Cuba in a rather prompt and quiet way. The Kremlin denied the other day that "organized Soviet combat units" have newly "arrived" in Cuba, contending that its military personnel have been on the island for 17 years and are only training Cubans. The United States is evidently inclined to accept this explanation, if American intelligence determines on a continuing basis that the units found to be operating in a combat mode last month are no longer operating in that mode. That would mean at the least that, in the State Department spokesman's words, the units would be shorn of the "associated equipment" that makes them "functionally able" for combat. It also might mean clarifying, by verifiable agreement, a gray area that may exist between training and combat modes.

Moscow would save face by not having to admit that it either activated a brigade or took one out. Poorly placed to suddenly start objecting to an old training mission, Washington could claim it had foreclosed a politically disturbing combat presence.

If events actually are working out in this way, there will be loud objections. From the left, some Americans and many foreigners will say that the United States has no good reason dictating the form of the Soviet military presence in Cuba.

But in fact the United States has excellent reason. Everything about Cuba is politically and strategically touchy, and has been for Fidel Castro's 20 years in power. The discovery and subsequent general awareness of a Soviet brigade come at a moment of fresh volatility in the Caribbean and of serious instability in overall Soviet-American relations and create a new and unsettling political fact. The United States need not apologize for a policy aimed at limiting new foreign military activity in its own back yard.

From the right, a good number of Americans are already suggesting that this is the time and place to draw the line against Soviet geopolitical expansion, and to compel Moscow to remove the troops. Some say this, by the way, ostensibly to help the administration ratify SALT, and others to kill SALT.

In fact, it is right and necessary to draw the line on a Soviet combat presence, however belatedly discovered and however currently innocuous. But is it sensible to take the long extra step of saying that the Soviet troops, even if shorn of combat capability, must be expelled? Acquiescence in continued Soviet training leaves intact the Cuban military activities supported by that mission. But neither the administration nor its critics have yet proposed a feasible and politically acceptable way to close them out.

The immediate problem is that of having found Soviet soldiers in a combat mode. This is what is unacceptable, and generally believed to be so. If the Soviets return their men to a training mode -- and surely American intelligence, now that it's been alerted, can keep track -- this affair can be put to rest. If the Soviets don't, that is another story.