A truly difficult question has vexed Washington since Israel invaded Lebanon a fortnight ago. Should the United States be guided first by its shock at the expanding purposes and the terrible, excessive human costs of the Israeli operation-- which still goes on? Or should it accept and try to exploit the political and perhaps also strategic openings created by it? To put it another way, can the United States reasonably expect to criticize Israel on one front and gain its cooperation on another? Or must it choose?

Secretary of Defense Weinberger went public on Sunday with his view that the United States should in the first instance come down hard on Israel in order to show Arab friends it does not condone Israel's "unilateral resort to military force." Secretary of State Haig, on the other hand, has tended to soft-pedal such criticism of Israeli policy, the better to be able to work with Israel to take advantage of Syria's and the PLO's disarray in order to rebuild a central government in Lebanon. President Reagan, receiving Prime Minister Begin yesterday, went with Secretary Haig.

Whether he was right to do so will depend on how his decision is put into effect over time. It would have been uncharacteristic of Mr. Reagan, and cruel to Israel, not to support the originally stated Israeli purpose of combatting border terror. It would have been equally uncharacteristic of Mr. Reagan, and cruel to Lebanon, not to support the lately revealed Israeli purpose of reconstituting "Lebanon for the Lebanese." An important reason the other Arabs reacted so mildly to the Israeli attack, after all, is that they knew there was no defense of Syria's and the PLO's earlier depredations. It would only compound the damage if an opportunity were lost to restore order in Lebanon and perhaps also to produce peace between Lebanon and Israel.

But the instinct behind Mr. Weinberger's critique is sound. He is expressing the widespread revulsion felt at Israel's tactics, which have cost it dearly in American opinion. It would have been much better if, yesterday, Mr. Reagan had associated himself directly and explicitly with Mr. Weinberger's passionate concern for the innocent victims of Israeli guns in Lebanon. He still should give voice to the common sense of outrage in this country.

Secretary Weinberger also believes Israel's conflicts with its neighbors ought to be handled by political and diplomatic means. True, no nation can be expected to deal with imminent threats of terror strictly by a slow political or diplomatic process. But Mr. Weinberger is on the right track. Israel faces continuing terror in some considerable measure because it has not done its share to treat legitimate Palestinian grievances; others, including Palestinians, have their own responsibility. The heart of the problem does not lie in Lebanon, but in the West Bank and Gaza, the Palestinians' natural home.

To judge by what was reported yesterday, Mr. Reagan still lacks the foggiest notion that this is the core issue. He appears to have swallowed uncritically Mr. Begin's line that a limited law-and-order problem was all that had to be dealt with. If this is so, the results are predictable: for the region a continuing condition of instability, for Israel a continuing estrangement from the Arab and Moslem worlds and for the United States a continuing difficulty in pursuing its many regional interests.