THE UNITED STATES is reacting with an appropriate attentiveness and calm to the Kremlin's delivery of Mig23 aircraft to Cuba. No public accusations against Moscow and Havana are being made, but the intelligence people are trying to figure out whether the planes are meant simply to improve Cuba's air defenses or to pay off Cuba for its cooperation with Soviet designs in Africa, or whether the planes are the kind of Mig23 that can be equipped to deliver nuclear weapons.

On the face of it, the latter suggestion is farfetched. If the Soviet Union is heading in that direction, an immense crisis lies ahead, and Moscow surely knows it. To be moving toward giving Cuba a nuclear capability would not only be a politically and militarily unacceptable violation of the Soviet-American understanding that ended the Cuban missile crisis of 1962; in that understanding, Moscow agreed not to put "offensive weapons" in Cuba. More important, it would mark a staggering change in Moscow's prudent 20-year policy of not permitting its allies to acquire their own nuclear weapons - weapons that would give an ally the means to work Moscow into a crisis that Moscow itself could not control. In the 1950s, the Soviets aided China's nuclear program. Once Peking and Moscow fell out, the Kremlin never went down that road again.

But if it seems farfetched that Moscow would let Cuba edge down the nuclear road, then there is no denying the political complications that would flow from a perception by the American public that Moscow was doing so. That was apparently what Defense Secretary Harold Brown had in mind when he suggested, in a memo to the president leaked to the Evans and Novak column, that the Mig 23 question was one of "high political sensitivity." The leak was a heavy breather, carrying ominous warnings that this case was about to blow the prospects for ratification of SALT. One can be forgiven for wondering if that was not precisely the purpose of the leak.

Already some political damage has been done. This gives the administration all the more reason to make a complete accounting to the public. If there is a real danger, it must be faced. If it is psychological warfare by opponents of a SALT agreement, that needs to be known, too. It remains to say only that the Kremlin, if its hands are clean, has it own good reason to help the administration demonstrate that this new "Cuban missile crisis" is a hoax.