In 1986, in the largest covert CIA operation since the Vietnam War, the U.S. sent $400 million worth of weapons to support a rebel group fighting in which country?

During the height of the Cold War, the Soviet Union's 1979 invasion of Afghanistan prompted a massive covert response from the United States. The CIA program, codenamed Operation Cyclone, was designed to arm and fund the local Islamic resistance fighters, known collectively as the Mujahideen. The American strategy was to turn the conflict into a costly and unwinnable quagmire for the Soviets, essentially giving them their own "Vietnam War."

By 1986, this support had escalated dramatically. The funding mentioned in the question was part of a huge increase in aid that included not just cash and basic arms, but highly sophisticated weaponry. Most famously, this was the year the U.S. began supplying the Mujahideen with FIM-92 Stinger missiles. These shoulder-fired, heat-seeking missiles were a game-changer, allowing the lightly-equipped fighters to effectively shoot down heavily armored Soviet helicopter gunships and transport planes.

This neutralization of Soviet air superiority was a critical turning point in the war. The operation was successful in its primary goal, contributing significantly to the Soviet Union's eventual withdrawal in 1989. However, the massive influx of weapons (Review) had complex long-term consequences, fueling a subsequent civil war and contributing to the instability that allowed for the rise of the Taliban.