Final answer:
The Soviets left Afghanistan due to a prolonged, costly conflict, high casualties, economic burdens, and lack of international support, compounded by the effective US-backed mujahideen resistance and the internal desire for reform under Gorbachev. The withdrawal also reflects an underestimation of Afghanistan's historical resistance to foreign invaders and the dissatisfaction with Soviet intervention efforts.
Step-by-step explanation:
The Soviets decided to leave Afghanistan for multiple reasons, many rooted in the challenges and unexpected consequences of their invasion and subsequent war efforts. Initially, they invaded to support the pro-Soviet regime and to prevent an Islamic rebellion that could lead to Western influence and the potential establishment of US missile bases along their border. However, their limited understanding of Afghanistan's resistance history, culture, and the complex political situation led to a long, costly war that the Soviet troops were not able to win decisively.
The United States played a significant role by supporting the mujahideen, providing the rebels with sophisticated weapons such as surface-to-air missiles that were effective against Soviet aircraft. This external support, combined with the strong Afghan resistance, made the war increasingly unsustainable for the Soviets. The economic burden of funding the war, along with the loss of life and the strategic blunder of being engaged in a war that became highly unpopular internationally, led to a drain of resources and a plunge in global prestige for the USSR. These factors, in conjunction with the desire to reform under the leadership of Mikhail Gorbachev, contributed to the Soviet decision to withdraw their forces from Afghanistan by 1989.
Following the Soviet withdrawal, the region experienced chaos and power struggles, which eventually led to the rise of the Taliban, creating a new set of challenges for the region and the world. All these factors demonstrate a monumental example of the complexities and unintended consequences of a superpower engaging in an unwinnable and costly conflict far from its borders.