Complex sentence: The Garam Chishma area became important during the Soviet invasion of Afghanistan because the Soviets were unable to stop the flow of arms and men back and forth across the Dorah Pass that separates Chitral from Badakshan in Afghanistan.
Split into simpler sentences: The Garam Chishma area became important during the Soviet invasion of Afghanistan. During the invasion, the Soviets were unable to stop the flow of arms and men back and forth across the Dorah Pass that separates Chitral from Badakshan in Afghanistan.