THE ADMINISTRATION has got itself into a nice mess over the Soviet combat brigade now reported to be in Cuba. The brigade does not seem the product of a recent "buildup," and in a way that is unfortunate. For the administration could then have insisted -- indeed, against what might have been some of its instincts to the contrary, it would have been compelled to insist -- that Moscow had done something provocative, sly and unacceptable. It could have rallied virtually the whole country behind a demand that the Russians back down, no matter the danger to SALT.

But, the administration asserts, the brigade is not newly emplaced, only newly discovered. The secretary of state says it is mostly a case of the United States having assembled a new intelligence jigsaw puzzle of a force that has been around since the early 1970s, if not the early 1960s. This raises some disturbing questions about American intelligence. It leaves Jimmy Carter in a fix. Any demand to Moscow to remove the brigade will surely be met by a not unreasonable rejoinder: why are you so excited about soldiers whose presence you have not even noticed, let alone protested, for what your own intelligence says is years and years? But if Carter does not press such a demand, he invites escalating attacks on his political standing and on his diplomacy (SALT) alike. For this he has chiefly his own record of uncertain international purpose to blame.

Mr. Carter, however, must deal with the situation as it actually exists, and the first element of the situation is concern about the brigade. In and out of the Senate, some extreme and transparently political things are being said but, discounting them, there remains strong and sensible reason for Americans to object to this increment of Soviet adventurism and power. Fortunately, the State Department now recognizes this, too. A few days ago, even while saying it didn't know what the purpose of the force was, it said the force posed "no threat to the United States," but now it agrees that the matter is "very serious" and it asserts that "the presence of this unit runs counter to long-held American policies." On this more satisfactory showing of urgency, the administration is entitled to a certain discretion as it plans what to do.

It was never possible or desirable for the Senate to take a precious lawyerly approach to arms control and to pretend SALT could be judged and put into effect in a context excluding the foreign and defense policies of the Soviet Union. That is not to say SALT is a reward that should be withheld from the Kremlin to spank it for its policy in Cuba. The treaty, if it is ratified, will have to be safe in circumstances like those now prevailing; the people who put those troops in Cuba are the same ones we are making the treaty with. If SALT has any value, it should be to reduce the mutual costs and risks of the sort of superpower elbow-wrestling going on now. The question has only been sharpened by the revelations concerning the Soviet Brigade.