As the clouds lower over American-Isareli relations, it is necessary to identify precisely the source of the storm. The best explanation we have seen is suggested on the opposite page by Abba Eban. The former Isareli foreign minister points out that in 1970 the Isareli government specifically agreed that U.N. Resolution 242 applied to the West Bank. Menachem Begin dissented at the time and resigned his Cabinet post. It meant little because that left him as no more than an opposition voice, and no real negotiating prospect loomed. Now, however, he is in power and there is a real negotiating prospect and his dissentmeans a great deal. It means that the essential minimal condition for enabling Egypt to resume negotiations on a settlement of its territorial issue with Isarel - the assurance that Jordon will have the same opportunity to negotiate a peace-for-territory exchange - cannot be met.

For a time, the Carter administration paid too little attention to Mr Begins's longtime 242 position. The United States was slow to realize, for instance, that the Begin plan for limited Arab "self-rule" in the West Bank was not a halfway house to a full or fuller political status but the signal of an intent to deny such a transition. The adminstration did not at once realize that the religious foundation of Mr Begin's position provided a special problem. Isarelis have necessarily high security standards for the West Bank. The way in which those standards will be met is a difficult point - but negotiable. Religious principles, on the other hand, make negotiations impossible. In addition, President Carter apparently was careless in his first descriptions of Mr. Begin's proposal. He did not realize that the Israeli leader would take his approval of it as an opening bargaining position as a definitive endorsement of the principle involved.

Those various tactical lapses may well have encouraged Mr. Begin to think his West Bank position was acceptable to Cairo and therefore to Washington. That misunderstanding also seems to underlie Mr. Begin's repeated - and erroneous - claim that it was a change of American policy that produced the deadlock so grimly evident in his talks here this week with President Carter. It was not. It was an American recognition of the continuity of Mr. Begin's personal beliefs and of the resultant discontinuity, as Mr. Eban notes, in Israeli government policy.

Mr. Begin is right to say that Egypt and, in its time, Jordan must make concessions for peace. He would unquestionably find firm American support in this respect if negotiations were to resume. But to rule out in principle a negotiated West Bank withdrawal, as he does, is to block the only path by which Egypt can be expected to return to the table. For the Arabs, the question of some sort of Arab sovereignty over some considerable part of the West Bank is also a matter of principle, and in their case they have the reinforcement not only of Resolution 242 but also of the Israeli government - pre-Begin. To go against that is, simply, to block the path to peace. That is what brought American officials to the reluctant judgment that until the question is resolved - and only Israelis can resolve it - there can be no further progress toward a Mideast settlement.