IT IS A FAIR question whether President Reagan should have bitten his tongue, as he did over the last two months in contained public expression of his feelings about the loss of civilian life in the Lebanon war. His suggestion on Friday that he was discreetly deferring to "the sensitivity of the negotiations" sounded pretty lame. Probably more often he was deferring to an expectation that the invasion might produce certain specific and desirable political results, shared in varying measures with Israel: quiet on the Israeli-Lebanese border, a new political deal in Lebanon, the breaking of the PLO's organized military and terror operations, a demonstration of Soviet regional irrelevance and a push to the long-stalled talks on Palestinian autonomy in the West Bank. Mr. Reagan would have done better to keep his political objectives front and center throughout the invasion period, if only to give a clearer focus to the debate over Israel's tactics in pursuing them.

Mr. Reagan seriously erred in being so sparing and "diplomatic" in his public comments. He overread the requirements of his political purposes. As a result, Israeli hawks found it easier to argue in Israeli councils either that the American administration agreed with their battering-ram policies or that no great price would be exacted if Israel continued them. Unquestionably, Arabs and others distort and exaggerate when they hold the United States responsible for all Israeli deeds of which they disapprove. But there is a measure of truth there all the same. The United States is not entirely without responsibility for Israeli deeds, especially in the Beirut context of day-after-day assaults. It would be inexcusable if President Reagan and other Americans were not to ponder hard whether the toll might have been reduced by more forceful expressions of American dismay.

It is argued that repetition would have dulled the point of protest. But that is only true if you believe that the Israelis would have ignored the protests and -- more important -- that Mr. Reagan would have idly stood by while the Begin government did. We don't believe it for a minute. Look at the Israeli response to the "outrage" Mr. Reagan finally voiced personally to Mr. Begin on Thursday, as Washington became aware that on that very day -- as the details of the PLO withdrawal from Beirut were being tacked into place -- Israel was conducting the most deadly attack of the war.

There are and will be various Israeli versions of this sequence, which involve possibly the most strenuous tasks of civilian control of the military in Israel's history. For Americans, the point remains that President Reagan's personal intervention was followed by a quick cease-fire and by an Israeli Cabinet decision to install new procedures for keeping the armed forces in a properly subordinate position.

In Israeli politics, opprobrium attaches to being seen to bend, on anything, to "American pressure." Defense Minister Ariel Sharon taxed Menachem Begin on this score after President Reagan telephoned the prime minister on Thursday. But that is not something that ought to slow down an American president who sees Israel, or for that matter any other friendly country, doing something that is believed contrary to American interests and values alike. In those conditions, "pressure" should be brought to bear, and no one should have any doubt about it.