A republic in western Asia, occupying the Tigris and Euphrates valley, north of Arabia. Mesopotamia, which corresponds with the modern area of Iraq, was the center of the Sumerian, Babylonian and Assyrian civilizations for thousands of years, until its conquest by Persia in the 6th century B.C. For the next 24 centuries, the region was ruled by a succession of foreign powers: Persians, Greeks, Parthians, Romans, Arabs, Mongols and Turks. In the early 16th century, it was conquered by the Ottoman Turks, and its first stamps were those of the Ottoman Turkish Empire. During World War I, Mesopotamia was occupied by British forces, and it became a British mandated territory in 1920. In 1921, a kingdom was established under Faisal I, son of King Hussein of Hejaz and leader of the Arab Army in World War I. Britain withdrew from Iraq in 1932, although it intervened during World War II to overthrow a pro-Axis ministry. In 1958, the monarchy was deposed, and a pan-Arab, pro-Soviet republic was established. The new regime nationalized most Iraqi industry and broke up large land holdings. Iraq maintained close ties with Syria, which is ruled by another branch of the same Baathist political party that overthrew the monarchy and with the Soviet Union. In 1973, Iraq sent troops to support Syria in its war with Israel. In 1975 it brutally repressed Kurdish nationalist agitation in the north. In 1978, relations with the Soviet Union cooled, and a number of communists were executed.