Final answer:
The USSR pulled out troops from Afghanistan when the Soviet economy was beginning to erode.
Step-by-step explanation:
When the Soviet economy was beginning to erode, the USSR pulled out troops from Afghanistan. The Soviet Union had intervened in Afghanistan in 1979 to support a communist government, but faced strong guerrilla resistance from Afghan rebels known as the Mujahideen. The costly and protracted conflict in Afghanistan was a significant contributing factor to the economic decline of the USSR.
Learn more about Soviet Union's troop withdrawal