IS IT SAFE to say that the Cuban "crisis" of 1979 may now disappear into the same mists of ambiguity from which it emerged, wraithlike, six weeks ago? Reasonable people everywhere must hope that Jimmy Carter's speech of Monday evening laid the thing to rest. A less substantial and less necessary crisis, and a potentially more disruptive one, is hard to recall.

This sequence opened -- feverishly -- with Sen. Frank Church's revelation that American intelligence had located a Soviet combat brigade in Cuba. The State Department then said it was the intelligence community's "unambiguous conclusion" that the brigade was in place. The principal facts seemed established; the argument concerned merely how to react. Over the next month, however, these "facts" sagged into the version rendered Monday by Mr. Carter: during the summer, intelligence produced "persuasive" evidence that Soviet training forces had been organized into a "combat unit" uncounted years ago. Would the reaction have been the same if the administration had made this more cautious reading in the first instance? If it had tended more carefully to the manner of the revelation? If it had solicited an explanation from Moscow before the overblown public revelation, accompanied by a demand for a Soviet backdown, had been made? Emerging as it did, the news was bound to alarm people who had no reason to doubt the stated propositions about what had happened, to be exploited by Mr. Carter's political rivals, and to feed the anxieties many Americans feel about whether the United States is responding properly to Soviet policy.

Having created this dilemma, Mr. Carter made a brave effort Monday to resolve it. He had 1) to explain that there was no real military crisis, 2) to deal with what was for him a real crisis of his own leadership, and 3) to preempt the real international crisis that would have arisen if, unable to cope with the political crisis, he had let SALT fail. We think he may have achieved all of these things by acting with balance and a sense of proportion and by asserting national determination without overheating the international air. Not to have taken such sensible steps as he did to deal better with contingencies in the Caribbean and elsewhere would have been to abdicate leadership. It would have been just as bad to have overreacted militarily or to have made demands -- the troops must go, SALT must be suspended -- that many Americans and others would have thought Moscow justified in resisting. The president left domestic critics and foreign adversaries grumbling, but business -- SALT and the defense budget -- can go on.

Jimmy Carter did not put his whole presidency to rights Monday, nor the country's whole strategic situation. At best he recouped some lost ground. One sigh has replaced another: a sigh of dismay that a crisis flared in the first place, followed by a sigh of relief that it can now be eased. The hard choices remain.