In 1978's, the Soviets initiated a coup to install communist leader in Afghanistan. Initially the communist leader installed was unpopular but nonetheless gained support in the major cities. Some parts of those cities became very modern and secular (a la the photo above). However, the vast majority of the country did not live in these secular havens and instead lived very traditional lives in rural Afghanistan. They were strongly Islamic and condemned the secular teachings of the communists. They put up resistance to the communist takeover, but were ill equipped to fight the Russian superpower due to lack of training, weapons, and an organized chain of command. This vague group of militia resistance fighters came to be known as the Mujhadeen ("Jihadi Warriors"). As numerous as they were, they were no match for the USSR and their communist allies in Afghanistan. The communist takeover seemed inevitable.
The USA of course opposed the global spread of communism but did not want to fight a direct war with the USSR. They noticed the strong but ineffective resistance of the rural Islamic Afghani's and decided to leverage their zeal to fight a proxy war with the communists. So, basically, the USA whipped up the Mujhadeen by providing them with money, weapons, and sometimes training. The war against the soviets was a holy war for the Mujhadeen and the USA used that to their advantage to create highly dedicated anti-communist soldiers.
The goal of the Americans was to do to the Soviets what the Soviets did to the Americans in Vietnam. That is, wrap them up in an unwinnable and costly war against a group of viciously dedicated guerrilla fighters.
Needless to say, it worked. After 9 years of hellish fighting, Soviet support for the war dwindled and they withdrew from Afghanistan. What was left of the communist government crumbled and the Mujhadeen, which was really made up of several otherwise unaffiliated tribes, started fighting each other for control of the countries resources and cities. Following the Soviet withdrawl, Afghanistan had no central government and was instead run by regional warlords, remnants of the Mujhadeen, which were funded through forced taxes and the opium industry.
Eventually, these warlords were ousted by the Taliban. Al-Queda, an ally of the Taliban commits the 9/11 attacks. Osama bin Laden, the leader of Al-Queda is hides out in Afghanistan. The Taliban refuse to turn him over to the USA. The USA invades Afghanistan and ironically starts fighting the remnants of the fundamentalists Muhjadeen they helped to create.
That's basically how the story goes but it extremely complicated. Books have been written just to explain the complexity of how Afghanistan came to be. It's not as simple as many people on here are making it out to be and I think, as voters, we have some responsibility to really understand this stuff.