"The President, Vice President and all civil Officers of the United States, shall be removed from Office on Impeachment for, and Conviction of, Treason, Bribery, or other high Crimes and Misdemeanors." - The U.S. Constitution.
After 9/11, Congress passed the "War Powers Act." It said:
“The president is authorized to use all necessary and appropriate force against those nations, organizations or persons he determines planned, authorized, committed or aided the terrorist attacks that occurred on Sept. 11, 2001, or harbored such organizations or persons.”
Bush was asked in January 2003 if there was a link between the Hussein government and 9/11. He replied: “I can't make that claim.”
The invasion of Iraq was illegal. You can read more about Bush's law-breaking here. You can sign a petition to impeach here. And you can really get the ball rolling by contacting your congressperson, and stumping for them to take action.
Lying about fellatio; lying about WMDs and 9/11? Both are lying. Both should receive the same response: impeachment.