JIMMY CARTER is ending the United States' status as a colonial power, and high time, too. His nagotiator, Peter Rosenblatt, has just initialed a "compact" by which three ministates in "free association" with the United States will be created out of the 2,000-island Pacific territory of Micronesia that the United States took from Japan in World War II. The decolonizing process has some further stages: negotiation of nuts-and-bolts subsidiary agreements; signature by four governments; plebiscites in Palau, Marshall Islands, and the Federated States of Micronesia; a joint resolution in Congress; and possibly some form of United Nations sign-off. But the hard part has been done.

Micronesia is not your ordinary colony. With its small size (100,000 people), immense area, lack of a shared past and culture, and economic and military vulnerability, it could not simply have been spun off into independent nationhood, even if the United States had wanted that -- the islanders never have. It took the United States 20 years, however, even to start negotiations on self-determination. Meanwhile, this country made colonial military use of the islands. Economically, it neglected them and then swamped them in the benefits of the welfare state.

The negotiations on the new compact produced intense conflict among Washington's security, diplomatic and welfare bureaucracies and their congressional partners. The terms show the strains. Protection is assured to American security interests; these include continuing missle testing in Kwajalein and keeping discreetly open a base option in Palau. The United States will run defense. In return, the islanderers will be able to choose their political future (and to change it in 15 years, even to full independence), to control their internal affairs and aspects of their foreign relations, and to receive generous economic benefits -- meant, by the way, to start weaning them from their unhappy dependence on the United States.

A fair deal? Some will question an agreement that produces only semi-independence for Micronesia and defense primacy for the United States. The Micronesians' answer must matter most. The islanders, fearing that Ronald Reagan might be more possessive than President Carter, had been eager to have the new agreement initialed before he took office. But key security-minded figures on the Hill, including some important Republicans, are aboard. It shall fall to Ronald Reagan to put the finishing touches on a tortuously achieved and historic agreement terminating the United States' role as a colonial power.