In 2000, a quarter century after he first served as secretary of defense, he was appointed again to the office by President George W. Bush. Rumsfeld was an advocate of a national ballistic missile defense shield and flexible military forces, and his efforts to transform and modernize the military made him the most significant defense secretary since Robert McNamara. He also, however, became noted for his blunt, sometimes undiplomatic public comments and statements, some of which alienated American allies, and for asserting his authority in a manner that alienated other government officials and some military officers. His standing was also hurt when the U.S. invasion of Iraq, which he strongly advocated and set the parameters for, failed to find weapons of mass destruction and led to an occupation that U.S. forces found more difficult than predicted, in part because they were inadequately prepared for it. The U.S. abuse of Iraqi prisoners, revealed in May, 2004, and the ongoing insurgent and sectarian violence in Iraq led to calls for his removal, but the president several times reaffirmed his support for Rumsfeld. He resigned in 2006 as the administration moved to change its strategy in Iraq to respond to the increasing insurgency there; Robert M. Gates succeeded him as defense secretary.
See biography by B. Graham (2009); J. Mann, Rise of the Vulcans: The History of Bush's War Cabinet (2004).