From the Wikipedia page 
The Trust Territory of the Pacific Islands (TTPI) was a United Nations trust territory in Micronesia (western Pacific) administered by the United States from 1947 to 1986.
The territory comprised the former South Pacific Mandate, a League of Nations Mandate administered by Japan and taken by the U.S. in 1944.
The TTPI entered UN trusteeship on July 18, 1947 and was designated a "strategic area" in its 1947 trusteeship agreement. As such, its formal status as a UN trust territory could be terminated only by the Security Council, and not by the General Assembly as with other trust territories. The United States Navy controlled the TTPI from a headquarters in Guam until 1951, when the United States Department of the Interior took over control, administering the territory from a base in Saipan.
On October 21, 1986, the U.S. ended its administration of the Marshall Islands District. The termination of U.S. administration of the Chuuk, Yap, Kosrae, Pohnpei, and the Mariana Islands districts of the TTPI soon followed on November 3, 1986. The Security Council formally ended the trusteeship for the Chuuk, Yap, Kosrae, Pohnpei, Mariana Islands, and Marshall Islands districts on December 22, 1990. On May 25, 1994, the Council ended the trusteeship for the Palau District, after which the U.S. and Palau agreed to establish the latter's independence on October 1.
The area is now divided into four territories: