Final answer:
The U.S. supported the Mujahideen to undermine Soviet influence in Afghanistan, providing arms and missiles to curb the spread of communism and engage the Soviets in a costly conflict, as part of the Cold War strategy.
Step-by-step explanation:
The United States sent aid to the Mujahideen in Afghanistan during the Soviet-Afghan War primarily as a strategic effort to counteract Soviet influence during the Cold War. This support included money, arms, and surface-to-air missiles, which were pivotal in neutralizing Soviet aircraft and shifting the balance of power. Under the Reagan Doctrine, the U.S. backed various Islamic rebel factions, aiming to bog down Soviet forces in a costly and demoralizing conflict analogous to the Vietnam War for the Americans, with little regard for the long-term aftermath. This policy of supporting any group fighting against communism, despite future implications, was a component of a broader geopolitical strategy to prevent the spread of communism and protect U.S. interests.