The withdrawal of U.S. military forces from Iraq was a contentious issue in the United States for much of the 2000s. As the war progressed from its initial invasion phase in 2003 to a nearly decade-long occupation, American public opinion shifted towards favoring a troop withdrawal; in May 2007, 55% of Americans believed that the Iraq War was a mistake, and 51% of registered voters favored troop withdrawal. In late April 2007 Congress passed a supplementary spending bill for Iraq that set a deadline for troop withdrawal but President Bush vetoed this bill, citing his concerns about setting a withdrawal deadline. The Bush Administration later sought an agreement with the Iraqi government, and in 2008 George W. Bush signed the U.S.–Iraq Status of Forces Agreement. It included a deadline of 31 December 2011, before which "all the United States Forces shall withdraw from all Iraqi territory". The last U.S. troops left Iraq on 18 December 2011, in accordance with this agreement.