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United Nations trusteeship ends, Commonwealth begins

After Japan's defeat in World War II, the Northern Marianas were administered by the United States pursuant to Security Council Resolution 21 as part of the United Nations Trust Territory of the Pacific Islands, which assigned responsibility for defense and foreign affairs to the United States as trustee. Four referendums offering integration with Guam or changes to the islands' status were held in 1958, 1961, 1963 and 1969. On each occasion, a majority voted in favor of integration with Guam, but this did not happen: Guam rejected integration in a 1969 referendum.

In the 1975 Northern Mariana Islands status referendum nearly 80% voted to become a commonwealth of the United States, and in 1977 over 93% approved the constitution of the CNMI.

The people of the Northern Mariana Islands decided in the 1970s not to seek independence, but instead to forge closer links with the United States. Negotiations for commonwealth status began in 1972 and a covenant to establish a commonwealth in political union with the United States was approved in a 1975 referendum. A new government and constitution came into effect in part on January 9, 1978 after being approved in a 1977 referendum. The United Nations approved this arrangement pursuant to Security Council Resolution 683. The Northern Mariana Islands came under U.S. sovereignty on November 4, 1986, and the islanders became US citizens. Also on November 4, 1986, the Northern Mariana Islands constitution became fully effective under the Covenant.

In May 1981, volcanic eruptions led to evacuation of the island of Pagan. Most residents of Pagan have not yet returned to Pagan due to ongoing volcanic activity.

In the 1960s and 1970s agriculture and ranching became an important activity with thousands of beef cattle, dairy cows, and hogs, as well as many crops such as pineapple. The food production became an important source of food supply for the Marianas region.

The Chamorro-Carolinian Language Policy Commission was created in 1982 to carry out policies in support of the Chamorro and Carolinian languages and cultures.

In December 1986, 20 percent of the homes on Saipan were destroyed by Typhoon Kim, trees were stripped of foliage, thousands of coconut trees were knocked down, roads were blocked, and there was no electricity or public water supply for weeks.

In April 1990, the inhabitants of the western coast of Anatahan were evacuated after earthquake swarms and active fumaroles indicated that an eruption might be imminent, but no eruption occurred at that time. A further earthquake swarm occurred in May 1992. The first historical eruption of Anatahan occurred in May 2003, when a large explosive eruption with a VEI of 4 took place forming a new crater inside the eastern caldera and causing an ash plume 12 km (7.5 mi) high which impaired air traffic to Saipan and Guam.