Iraq was hardly a "response to 9/11". It'd been a festering sore of American foreign policy for over a decade

http://en.wikipedia.org/wiki/Iraq_War_and_U.S._Global_War_on_Terror

"At the outset of the war, the U.S. Congress and public opinion supported the notion that the Iraq War was part of the global war on terror. The 2002 Congressional resolution authorising military force against Iraq cited the U.S. determination to "prosecute the war on terrorism", and in April 2003, one month after the invasion, a poll found that 77% of Americans agreed that the Iraq War was part of the War on Terror"

So, yeah, the war on Iraq was very much a response to 9/11, in the sense of being sold as being part of the same "global war on terror" that was supposedly launched in response to 9/11.