THE BOMBS started going off in Manila last August. Several groups in a position to make their prophecy come true had warned that violence would be the sure result of the continuance of martial law. Notwithstanding a warning to stay away, an association of American travel agents chose Manila as a convention site. The strongman, President Marcos, boasted in his welcome that he had everything under control. On cue, a bomb exploded, wounding seven Americans and a dozen Filipinos. President Marcos has responded by ordering the arrest of some 30 political rivals, including a number of exiles living in the United States.

What stands out in this sequence is the utter familiarity and predictability of it all. For years, while President Marcos was consolidating a personal dictatorship that has made his one of the richest families in the world, it has been plain that he would eventually have to cope with the challenge of terror. There was another cycle in the early 1970s. Political rivals and some officials and citizens of the United States have long advised him to return to the country's traditional democratic ways. Mr. Marcos, who runs a military show but who is not without political skills, has even taken some tentative steps in this direction. But the emphasis is on the tentative. And now terror, the characteristic political medium of the age, is being employed to strike directly at the Philippines' crucial link with the United States.

Perhaps it ia premature to suggest that this is the beginning of the end for President Marcos. He will have something to say about that, in any event. It is even faintly conceivable that he could surprise the skeptics and deepen, rather than torpedo, the Philippines' movement back toward democracy.

But Americans will have choices of their own. The United States has stayed close to President Marcos largely for reasons of habit and Pacific strategy. To the extent that his hold is challenged, the United States will face the question that has dogged its post-war policy in the Third World: whether to support the status quo or seek and adjustment to change. As the Iranian example freshly proves, either course holds perils. Mr. Marcos could do himself and his American friends a world of good by deciding himself to end personal rule.