"A situation in Asia that was not without considerable promise for the USA was to become far more threatening in 1979. The overthrow, in the face of mass-demonstrations, of the Shah of Iran, who left Tehran on 16 January 1979, and his replacement by a theocratic state hostile to the USA, combined with the Soviet invasion of Afghanistan at the end of 1979 to create a highly volatile situation that posed problems for analysts. There was the prospect, first, that the USA might lose the struggle for regional hegemony and, secondly, that this might have wider consequences across Asia. Although authoritarian and prone to initiatives that were not always welcome, Iran was America’s leading ally in South Asia, an opponent to Arab radicalism, and a block to Soviet expansionism and that of Iraq, a key Soviet ally. The USA had used Iran to support the Kurds against Iraq, and, in 1973, to send troops to help the Sultan of Oman overcome left-wing rebels based in the region of Dhofar, rebels backed by Communist powers. Iran was also a major purchaser of American arms, and a key oil exporter. Under the Shah, it had long played a central role in the forward containment of the Soviet Union, not least by providing important radar bases to screen the southern Soviet Union. After the collapse of Iran, the Americans fell back upon Israel."