FOR 26 YEARS Israel has acquired arms under a formal agreement with the United States. On occasions too numerous to recite it has used those weapons outisde as well as inside its own borders. Through three major wars and countless incidents, successive administrations have understood that the realities of Arab hostility justified the reading that, in using American weapons in that way, Israel was acting in legitimate self-defense. Why, then, did the secretary of state feel bound to slap Israel hard on Wednesday by informing Congress that Israel "may" have violated the 1952 military-aid agreement in its recent intervention in Lebanon?

Mr. Vance offered no explanation. He merely made the charge and indicated Washington would not invoke the arms cutoff that a 1976 law requires if a "substantial violation" of a military-supply agreement is found. But that is unsatisfactory. What are the standards the State Department applies, in this case and in general? All arms recipients are entitled to know. Is it conceivable that "legitimate self-defense," the 1952 language, does not cover a country that has lost more than 1,000 lives to terrorists crossing from a neighboring state - Lebanon - that does not perform the elementary national duty of policing its own territory? A county that, even as the rebuke arrives, is withdrawing its forces under the terms of a U.N. resolution drafted in the first instance by the United States? We don't understand.

We support the administration's strategy of seeking a Mideast peace agreement. But some of its tactics are insensitive. Right now, for instance, the administration should be concentrating on persuading Israel to rethink parts of its position in the negotiations with Egypt. It can only weaken the American case if, while presenting it, the United States acts in other ways to embarrass Israel and nourish its swollen suspicions that its security is of fading American concern. It is one thing to criticize the Lebanese operation, as we have done, for its heavy civilian toll. It is another to smear Israel with a vague allegation that puts a cloud over access to the arms on which its security depends. As a good lawyer, Mr. Vance could have found a half-dozen other ways to handle the congressional inquiries on Israel's use of American arms in Lebanon. Instead, a calculated decision seems to have been made to use the issue as a gratuitous political goad.