The United States, which willingly or not is a decisive actor here, appears headed for a test of agility over the crumbling Duvalier dictatorship that has been a reliable if distasteful Caribbean ally.

For an administration that has criticized president Carter over his handling of Iran and Nicaragua as their U.S.-allied dictators fell from power, the slide of Jean-Claude Duvalier in Haiti seems likely to join the Philippines in providing an easily observable chance to do better.

This little island is not strategic Iran, of course. And as far as anybody knows, there are no Haitian Sandinistas in the hills to reorient the nation toward Cuba. The stakes here, according to Haitian and diplomatic assessments, seem instead to be fostering a transition from repressive dictatorship without setting off the mob violence that repeatedly has marked Haitian history since independence from France in 1804.

The U.S. military occupied the country from 1915 to 1934 because of one episode in that instability, and president Kennedy had a Marine-bearing ship sailing off the coast 23 years ago because of violence directed against Duvalier's father, Francois (Papa Doc) Duvalier.

Geography also gives Washington an interest in what happens here. The island lies just across the Windward Passage from Cuba and 600 miles from the shores near Miami, where thousands of Haitian "boat people" have tried to slip into the United States.

Although the 34-year-old president-for-life has vowed to stay on, U.S. officials say that they regard his downfall as increasingly likely, even inevitable over time.

Some Haitians who support Duvalier suggest that the premature White House announcement of his fall yesterday betrayed an attempt to promote his departure. Others who oppose him say U.S. tolerance is largely responsible for the dictatorship's survival so far. In any case, foreign observers note, the White House episode indicates a predisposition in Washington to believe Duvalier is not long for the world of despots.

Officially, U.S. policy here dictates that the Haitian people must decide for themselves whether Duvalier stays on at the head of the regime founded 29 years ago by his father and inherited in 1971. More realistically, however, the Haitian people have little say in government and seem able to express themselves only in street violence.

Some Haitians and diplomats believe that the United States already has played a large role in fomenting antigovernment unrest by insisting on human rights reform as a condition for U.S. aid. Partly as a result of the U.S. presssure, the young Duvalier tolerates dissent to a degree that his iron-fisted father would never have imagined.

Today's edition of the weekly "Petit Samedi Soir," for example, declares that the recently disbanded political police "in a better oiled dictatorship would be the SS of Heinrich Himmler." That was written in the French understood by only about 10 percent of Haiti's 6 million inhabitants. But without the U.S. human rights pressure, it probably would not have been written at all.

More decisive, in the eyes of informed observers here, is the Creole language broadcasting of the Catholic Church's Radio Soleil. Until the government shut it down again in yesterday's crackdown, Radio Soleil and a companion Protestant station were the most widely listened to on the island. Similarly, Duvalier was largely responding to U.S. aid conditions in allowing these reports of agitation to continue as long as they did.

Duvalier's orders in 1984 forbidding security forces to torture or beat prisoners, as well as his largely discredited elections two years ago, also are seen here as part of his attempts to meet U.S. human rights demands. Although observed largely in the breach, the idea of restraints on police and the Ton-tons Macoutes political enforcers is seen as a decisive factor in the increasing willingness of crowds to take to the streets to vent their frustrations.

U.S. diplomats explained the human rights policy as an effort to induce reforms within the hereditary president-for-life system.

But Haitian officials and other diplomats here say that forcing even a little relaxation here was almost inevitably going to loosen pent-up dissatisfaction and irreversibly erode government control like water expanding a hole in an earthen dam. One indication that Haitians believe this is the U.S. flags that occasionally have been seen waving among the crowds during this week's street violence in provincial cities.

The Reagan administration is moving toward a deadline of sorts for what is likely to be interpreted here as another indication of U.S. attitudes. That is the need to certify to Congress for 1986 that Duvalier's government is making human rights progress as a condition for approval of the administration's request for $56 million in aid.

The certification, already doubtful, now seems still more remote given the week's events, which culminated in a state of siege. This sets the stage for a symbolic but public slap at Duvalier's government.

The Haitian armed forces, most often cited as the likely instrument of change, receive as U.S. aid only Coast Guard help for the Navy and individual officer instruction in the United States, U.S. officials say. Despite this limited U.S. involvement, many educated Haitians say the United States would have to bestow a sort of blessing on any officers who tried a coup d'etat, because of traditional U.S. influence here.