The Soviet Union became the ruling power in the early 1920s and expelled Mohammed Alim Khan. It later put down the Basmachi movement and killed Ibrahim Bek. A large refugee population of Central Asians, including Turkmen, Tajiks and Uzbeks, fled to northern Afghanistan. In the 1960s and 1970s the Soviets started using the Amu Darya and the Syr Darya to irrigate extensive cotton fields in the Central Asian plain. Before this time, water from the rivers was already being used for agriculture, but not on this massive scale. The Qaraqum Canal, Karshi Canal, and Bukhara Canal were among the larger of the irrigation diversions built. However, the Main Turkmen Canal, which would have diverted water along the dry Uzboy River bed into central Turkmenistan, was never built. The 1970s, in the course of the Soviet–Afghan War, Soviet forces used the valley to invade Afghanistan through Termez. The Soviet Union fell in the 1990s and Central Asia split up into the many smaller countries that lie within or partially within the Amu Darya basin.