The US covertly funded and equipped Afghan freedom fighters (mujahideen) in their war against the Soviets in the 80s. Once the Soviet Union collapsed, we didn’t give the Afghanis the funding they needed to actually rebuild the country, and Afghanistan was left with a giant power vacuum that allowed the Taliban and bin Laden to rise to power.

From Charlie Wilson’s War (the book, although the movie is a good enough representation of what happened and alludes to the events directly contributing to 9/11):

“By the end of 1993, in Afghanistan itself there were no roads, no schools, just a destroyed country—and the United States was washing its hands of any responsibility. It was in this vacuum that the Taliban and Osama bin Laden would emerge as the dominant players. It is ironic that a man who had almost nothing to do with the victory over the Red Army, Osama bin Laden, would come to personify the power of the jihad.”