DID MR. BEGIN think, as he annexed the Golan Heights the other day, that President Reagan was too preoccupied with Poland to mind? If so, Mr. Reagan must have disabused the Israeli prime minister of that notion yesterday. Mr. Reagan suspended the memorandum on strategic cooperation that the two nations signed and that the Begin government hailed as a signal breakthrough only a few weeks ago. He cut off discussions on Israeli proposals for indirect but substantial further American financial support. This is strong stuff, and it should help dispel the giddiness that the annexation has produced in Israel. It is not pleasant for the United States to apply direct and open pressure on a friend, or to contemplate having to apply more. Mr. Begin made it unavoidable.

The United States had supported a unanimous Security Council resolution terming the annexation null and void. In its statement yesterday, the administration added a note of its displeasure that Israel moved while the Polish crisis was unfolding. It complained that Israel had not acted in the spirit of mutual consideration embodied in the memorandum on strategic cooperation. All this is so, but it does not go to the heart of what was wrong with Israel's unilateral act.

The act undercut the formally agreed basis, the only conceivable basis, on which peace can come to the Middle East: the yielding of territory by Israel in return for peace agreements with its neighbors. With Egypt, Mr. Begin did agree to exchange territory for a treaty. With the Golan beyond even theoretical reach, however, Syria has no reason to accept Israel and negotiate for the Golan's return; other nations have no reason to press Syria to do so. Is Mr. Begin's plan to foreclose negotiations on the West Bank by unilaterally annexing that sensitive territory, too?

Israel already had ample international difficulties, not all of its own making. By this one stroke, however, Menachem Begin has forfeited the solicitude that had flowed to Israel as a result of the death of Anwar Sadat and the AWACS deal; converted Syria, a radical, terror-minded Soviet client state, into at least a temporary diplomatic partner of the United States; perplexed and embarrassed Israel's greatest friend and only patron, the United States; and deepened his own country's terrible isolation. Mr. Begin showed himself capable of statesmanship in making peace with Egypt. Israel needs his statesmanship now, desperately.