President Reagan's speech on peace at the United Nations yesterday was happily complemented by his audience's awareness that he had just made a noticeable contribution to bringing about a cease-fire in Lebanon. It was one of those all-too-infrequent occasions in international politics when a nation's actions give a boost to its words.

It is, of course, far too early to celebrate success in Lebanon; and it needs also to be remembered that the terrible troubles Mr. Reagan and others are addressing there were created and exacerbated by the defaults and errors of many countries--our own included. The violence has been suspended for an uncertain time during which the Lebanese have another chance to deal with problems whose solutions have eluded them for years. Too many times before, the Lebanese and their friends have pronounced "enough" to the killing, only to be pitched back into new bloodletting.

For at least the moment, however, the American contribution can be commended. Mr. Reagan had little warm company and much hot criticism in undertaking to use the force necessary to support his diplomacy. He had to move against a wily and vengeful Syrian regime stiffened by Soviet arms and men. While bolstering an infirm Lebanese government, moreover, he had to bring it to accept a commitment to an exceedingly painful exercise in power sharing. Just to traverse this mine field with Saudi Arabia, the United States' indispensable regional partner, was an achievement.

The resulting cease-fire is much more than a cease-fire. It is a road map of sorts to a "new Lebanon"--a Lebanon unoccupied by foreign armies and at peace with itself. There can be no doubt that no strictly military agreement, no agreement not setting up a new political process, could have won Lebanon even a temporary reprieve. But it is not too early to begin asking, quietly, just how far the United States intends to accompany Lebanon down what promises at best to be a very long and tortuous road. Specifically, what is the tenure and the role of the Marines? What possibilities now exist for a handover to the U.N.? Mr. Reagan cannot put off such difficult questions for long.