THE UNITED STATES quickly, and creditably, "condemned" Israel's attack on the Iraqi nuclear reactor. In the same breath it said it was looking into the question of whether Israel had used American arms in violation of the defensive purposes for which they were sold. This is the right question, no matter that it offends Israelis. It would be irresponsible to provide weapons to any nation without attaching at least loose strings, and it would be even more irresponsible to disregard them once attached.

Mr. Reagan came to power, you will recall, flaying his predecessor for having been an inconstant ally. The basic Reagan theory of foreign policy centers on distinguishing sharply between the treatment of friends and foes. Yet there he is, a few months in office, embroiled in public disputation with a friend to whose welfare and security he has pledged himself a hundred times. Israel forced this upon him. Mr. Reagan has discovered that friendship is no guarantee of perpetual common interest and that clients can take patrons where they do not wish to go. To acknowledge this is to court political and diplomatic embarrassment now. Not to acknowledge it is to invite a larger disaster later.

It is suggested that Congress may not care to rebuke Israel for a raid that had, after all, a certain rationale. Is if further suggested that the administration realizes the immediate tactical value, in its separate approaches to Arabs and Israelis, of making a show of disapproval. But Israel had directly challenged the fundamental terms on which the United States conducts its Mideast policy. The United States must be in a position to pursue its interests with Arabs as well as Israelis. An Israeli policy that thwarts that purpose cannot be accepted. This is reason enough for the president to press the arms issue with the Israelis no matter what Congress does.

That's not all. The success of the raid and the international reaction to it, leading citizens to rally around the government, could bolster Prime Minister Begin's bid for reelection next month, though the extravagance of his act and the evidence -- in the American criticism -- of his mishandling of the crucial American connection could take their own political toll. Regardless of who is elected, it is essential that no one be left in doubt about where Mr. Reagan stands. Forget about further progress toward an Arab-Israeli settlement: the administration can expect redoubled resistance to its efforts to knit an anti-Soviet consensus as long as Arabs feel that, by wink or word, Mr. Reagan has licensed Israel to shoot at will.