Well, we went in to go after bin Laden and his al Qaeda followers who had been hiding out in the mountains of Afghanistan and also traveling between there and Pakistan. The trouble is that we had to side with the Taliban opponents in order to be able to set up shop and work from there, rather than just go after bin Laden and al Qaeda via satellite and air strikes from other nations, which wouldn't have worked.
As for changing culture, we did change the Japanese culture to a modern thriving free-market capitalist nation, but unlike Afghanistan, their religions, Shinto and Buddhism aren't violent and the Emperor was a main guide in the nation and became more of just a figurehead after WWII.
Islam is the most violent and barbaric religion still in existence and will unlikely ever change, especially in Afghanistan and because of the estimated 320,000,000 Islamists on Earth that want all non-believers converted, subjugated or killed we will never know true peace with them.