The Authorization for Use of Military Force Against Iraq Resolution of 2002 was passed by congress with Democrats voting 58% in favor in the Senate, and 39% in favor in the House. Republicans supported the joint resolution 98% and 97% in the Senate and House respectively. The resolution asserts the authorization by the Constitution of the United States and the Congress for the President to fight anti-United States terrorism. Citing the Iraq Liberation Act of 1998, the resolution reiterated that it should be the policy of the United States to remove the Saddam Hussein regime and promote a democratic replacement.

The resolution "supported" and "encouraged" diplomatic efforts by President George W. Bush to "strictly enforce through the U.N. Security Council all relevant Security Council resolutions regarding Iraq" and "obtain prompt and decisive action by the Security Council to ensure that Iraq abandons its strategy of delay, evasion, and noncompliance and promptly and strictly complies with all relevant Security Council resolutions regarding Iraq." The resolution authorized President Bush to use the Armed Forces of the United States "as he determines to be necessary and appropriate" to "defend the national security of the United States against the continuing threat posed by Iraq; and enforce all relevant United Nations Security Council Resolutions regarding Iraq."

The legality of the invasion of Iraq has been challenged since its inception on a number of fronts, and several prominent supporters of the invasion in all the invading nations have publicly and privately cast doubt on its legality. It is claimed that the invasion was fully legal because authorization was implied by the United Nations Security Council. International legal experts, including the International Commission of Jurists, a group of 31 leading Canadian law professors, and the U.S.-based Lawyers Committee on Nuclear Policy, have denounced both of these rationales.

On Thursday November 20, 2003, an article published in the Guardian alleged that Richard Perle, a senior member of the administration's Defense Policy Board Advisory Committee, conceded that the invasion was illegal but still justified.

The United Nations Security Council has passed nearly 60 resolutions on Iraq and Kuwait since Iraq's invasion of Kuwait in 1990. The most relevant to this issue is Resolution 678, passed on November 29, 1990. It authorizes "member states co-operating with the Government of Kuwait... to use all necessary means" to (1) implement Security Council Resolution 660 and other resolutions calling for the end of Iraq's occupation of Kuwait and withdrawal of Iraqi forces from Kuwaiti territory and (2) "restore international peace and security in the area." Resolution 678 has not been rescinded or nullified by succeeding resolutions and Iraq was not alleged after 1991 to invade Kuwait or to threaten do so.

Resolution 1441 was most prominent during the run up to the war and formed the main backdrop for Secretary of State Colin Powell's address to the Security Council one month before the invasion. According to an independent commission of inquiry set up by the government of the Netherlands, UN resolution 1441 "cannot reasonably be interpreted (as the Dutch government did) as authorising individual member states to use military force to compel Iraq to comply with the Security Council's resolutions." Accordingly, the Dutch commission concluded that the 2003 invasion violated international law.

At the same time, Bush Administration officials advanced a parallel legal argument using the earlier resolutions, which authorized force in response to Iraq's 1990 invasion of Kuwait. Under this reasoning, by failing to disarm and submit to weapons inspections, Iraq was in violation of U.N. Security Council Resolutions 660 and 678, and the U.S. could legally compel Iraq's compliance through military means.

Critics and proponents of the legal rationale based on the U.N. resolutions argue that the legal right to determine how to enforce its resolutions lies with the Security Council alone, not with individual nations.

In February 2006, Luis Moreno Ocampo, the lead prosecutor for the International Criminal Court, reported that he had received 240 separate communications regarding the legality of the war, many of which concerned British participation in the invasion. In a letter addressed to the complainants, Mr. Moreno Ocampo explained that he could only consider issues related to conduct during the war and not to its underlying legality as a possible crime of aggression because no provision had yet been adopted which "defines the crime and sets out the conditions under which the Court may exercise jurisdiction with respect to it." In a March 2007 interview with the Sunday Telegraph, Moreno Ocampo encouraged Iraq to sign up with the court so that it could bring cases related to alleged war crimes.

United States Ohio Congressman Dennis Kucinich held a press conference on the evening of April 24, 2007, revealing US House Resolution 333 and the three articles of impeachment against Vice President Dick Cheney. He charges Cheney with manipulating the evidence of Iraq's weapons program, deceiving the nation about Iraq's connection to al-Qaeda, and threatening aggression against Iran in violation of the United Nations Charter.