Why only Afghanistan?

Oil drop effect. Start with Iraq. You spread democracy and in a democracy it is much more difficult for terrorists to train and thrive. Your economy also improves and people demand more and more rights for themselves. Before you know it you have a fully functioning democracy right in the middle of the middle east. You think Iranians will stand for that? Iranian students without the Iraq factor have been protesting for decades against their oppressive government. They will demand more. And it goes on and on. Country after country becomes less of a safe haven for terrorists.