Intelligence
Afghanistan was the country (Governed by the Taliban) where Osama bin Laden was living during and after the 9 11 attacks. america demanded that the Taliban extradite bin Laden to face trial on charges of terrorism and the Taliban quite rightly asked "do you have any evidence that he has commited any crimes." They futher said that if we had any valid evidence they would extradite him. We of course had no evidence or at least refused to show it if it existed and thought it would be easier to invade and over throw the government. It was of course no more than an excuse to build a pipeline.