In late 1971, the rival African factions working for black-majority rule in white-controlled Rhodesia (Zimbabwe) formed a coalition, the Front for the Liberation of Zimbabwe, which beame a joint guerrilla effort to overthrow the government. The black guerrillas operated from bases in Zambia and from FRELIMO-controlled areas in Mozambique and made periodic raids in Rhodesia. With the collapse of the Portuguese empire in Africa in 1974-76, Rhodesia's Prime Minister Ian Douglas Smith (1919-) saw his country surrounded on three sides by unfriendly African nations and declared a state of emergency to combat the rebel guerrillas. Mozambique closed its 800-mile-long border with Rhodesia, but Rhodesian government troops frequently violated it in "hot pursuit" of guerrillas. In 1976, Rhodesian soldiers destroyed a United Nations refugee camp, declaring it was hiding guerrillas. Black antigovernment rebels also operated from three fronts in Zambia. As the violence increased on both sides, the United States and Great Britain tried to negotiate a peaceful settlement; white Rhodesians, however, were unwilling to relinquish political and economic power, and the black Africans were divided by tribal, ideological and political differences. In 1978, guerrillas attacked one of Rhodesia's main cities, Umtali, with mortar fire; in retaliation, the Rhodesian army bombed bases 125 miles within Mozambique. Finally, in 1978, an agreement was reached on a constitution to transfer power to the black majority. The country's name was changed to Zimbabwe Rhodesia, and in the general election (April 24, 1979) Bishop Abel Muzorewa (1925-) became the country's first black prime minister when his party won more than 67 percent of the vote. However, the two powerful black nationalist factions led by Joshua Nkomo (1917-) and Robert Mugabe (1924-) denounced the agreement and the results and continued fighting. In the fall of 1979, Britain called a peace conference in London to which all African leaders were invited. Eventually a new agreement was hammered out that was acceptable to everyone. The UN economic sanctions imposed on Rhodesia since 1966 were lifted in late 1979. In elections in 1980, Mugabe was chosen prime minister, receiving almost 63 percent of the popular vote. Britain handed the reins of government to him; the new republic's name was officially shortened to Zimbabwe.