It depends on whether they created the situation...or simply facilitated it, Ebbie, but here's why it was Afghanistan.

The USA had been negotiating with the Taliban in the summer of 2001 and had offered them (off the record) this choice: "a carpet of gold or a carpet of bombs". Over what? Over the fact that the USA wanted to move in American contractors to build a major oil pipeline through Afghanistan in order to move oil from the Caspian region through to the Indian Ocean. The Taliban said "No". They refused to cooperate. That was when the decision was made to invade Afghanistan. From that point on it was necessary to create media coverage that would motivate the American people to support an unprovoked war on Afghanistan. Various stories had been in the media already, like the destruction of those ancient Buddhist statues and the mistreatment of Afghan women, but none of those stories were big enough to get the American people to back a war. Something much bigger was needed.