Map of a divided Yemen, pre-1990
Yemeni unification occurred on 22 May 1990, when the socialist state of South Yemen and the nationalist North Yemen united to form a pan-Yemeni republic of Yemen. The two Yemens had historically been separated, and were not the product of war or occupation; North Yemen had evolved from the Mutawakkilite Kingdom of Yemen, which had gained its independence from the Ottoman Empire in 1918, while South Yemen gained its independence from the United Kingdom in 1963. North Yemen was a republican government that included Sunni Muslim tribal representatives, while South Yemen was a secular and socialist state. Yemen was the only Marxist nation in the Middle East during the Cold War, and it received economic assistance from the Soviet Union. In 1972 and 1979, the two Yemeni governments briefly went to war with each other, but the discovery of oil during the 1980s led to both governments cooperating. In 1989, the Yemeni governments began the process of drafting a constitution for a united Yemen, and the country was officially united on 22 May 1990. However, friction between the Shia south and the Sunni north would endure into the 21st century, leading to the 1994 civil war and the 2015 civil war.