FROM THE STATIC that arose from his remarks on nuclear war the other day, President Reagan should have taken a valuable lesson: talking about nuclear war is a political mine field, especially when, as now, deep currents of anxiety about official policy are running in this country and abroad. Most days it just might be better to say that the United States is determined to defend its allies and deter its adversaries and let it go at that. Any effort to stray beyond this familiar terrain invites trouble.

So it was when Mr. Reagan, who knows what he thinks on the matter but is painfully unpracticed at expressing it, permitted himself to ramble on about nuclear war last Friday. Along the way he said, with his characteristic candor, that he didn't honestly know if a limited nuclear exchange would escalate, but that in a stalemate "I could see where you could have the exchange of tactical weapons . . . without it bringing either one of the major powers to pushing the button."

There was nothing new or surprising about this statement. Almost inevitably, however, the European press, which is hyperthyroid on this issue, picked it up. It converted his unexceptionable observation--that a limited exchange might not escalate--into an indiscreet confession that the United States would allow Europe to become a nuclear battlefield while Americans perched safely on the sidelines. By the time the sequence had run its course, Leonid Brezhnev was demanding that Mr. Reagan make a "clear and unambiguous statement" rejecting the very idea of nuclear attack.

Actually, a "clear and unambiguous statement" is, for better or worse, out of the question. The American doctrine governing use of nuclear weapons has always had a central core of ambiguity, and properly so. On the one hand, the United States wants to persuade Moscow, in the words of a second Reagan statement issued Wednesday, "that no aggressors should believe that the use of nuclear weapons in Europe could reasonably be limited to Europe." On the other, the United States wants to assure Europeans that if it were necessary to use nuclear weapons in their behalf against a Soviet attack, they would not be used promiscuously. The first line requires Washington to communicate a deadly intent and the second a sense of restraint. This is the heart of the nuclear paradox. It is unavoidable, and it lends itself to confusion in the best of times.

And these are not the best of times. One reason is that Mr. Reagan's hard-line approach to the Soviet Union has fed already-existing anti-NATO, anti- nuclear and even anti-American currents in Europe and has complicated the work of European governments in keeping Atlantic relations on an even keel. Since the margin of common interest and understanding on which American leadership in NATO rests is not unlimited, Mr. Reagan cannot afford to expend American capital loosely. He should be careful to release no more nuclear static into the air.