That's untrue. I quote from the Iraq Resolution, the document passed by Congress declaring war:

Whereas members of al Qaida, an organization bearing responsibility for attacks on the United States, its citizens, and interests, including the attacks that occurred on September 11, 2001, are known to be in Iraq;

Whereas Iraq's demonstrated capability and willingness to use weapons of mass destruction, the risk that the current Iraqi regime will either employ those weapons to launch a surprise attack against the United States or its Armed Forces or provide them to international terrorists who would do so, and the extreme magnitude of harm that would result to the United States and its citizens from such an attack, combine to justify action by the United States to defend itself;

Whereas the attacks on the United States of September 11, 2001, underscored the gravity of the threat posed by the acquisition of weapons of mass destruction by international terrorist organizations;

Whereas the United States is determined to prosecute the war on terrorism and Iraq's ongoing support for international terrorist groups combined with its development of weapons of mass destruction in direct violation of its obligations under the 1991 cease-fire and other United Nations Security Council resolutions make clear that it is in the national security interests of the United States and in furtherance of the war on terrorism that all relevant United Nations Security Council resolutions be enforced, including through the use of force if necessary;

Whereas Congress has taken steps to pursue vigorously the war on terrorism through the provision of authorities and funding requested by the President to take the necessary actions against international terrorists and terrorist organizations, including those nations, organizations, or persons who planned, authorized, committed, or aided the terrorist attacks that occurred on September 11, 2001, or harbored such persons or organizations;

Whereas the President and Congress are determined to continue to take all appropriate actions against international terrorists and terrorist organizations, including those nations, organizations, or persons who planned, authorized, committed, or aided the terrorist attacks that occurred on September 11, 2001, or harbored such persons or organizations;

Whereas the President has authority under the Constitution to take action in order to deter and prevent acts of international terrorism against the United States, as Congress recognized in the joint resolution on Authorization for Use of Military Force (Public Law 107-40); and

I estimate that around half of the clauses in the preamble, which were official justifications given by the US for the war, deal with international terrorism, of which around a half mention al-Qaeda or 9/11.

Also, more broadly, do you really think we would have invaded Iraq without the hyper-jinoistic atmosphere caused by 9/11? (Remember, Bush campaigned in 1999 as an isolationist, who wanted to end Clintonian nationbuilding.)

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.