The legality of the Iraq War is a contested topic that spans both domestic and international law. Political leaders in the US and the UK who supported the invasion of Iraq have claimed that the war was legal. However, many legal experts and other world leaders have argued that the war lacked justification and violated the United Nations charter.

In the UK, John Chilcot, chairman of the Iraq Inquiry, concluded that the process of identifying the legal basis for the invasion of Iraq was unsatisfactory and that the actions of the US and the UK undermined the authority of the United Nations. John Prescott, Deputy Prime Minister to Tony Blair, has also argued that the invasion of Iraq lacked legality. In a 2005 paper, Kramer and Michalowski argued that the war "violated the UN Charter and international humanitarian law".

Russian President Vladimir Putin stated that the war was unjustified. In a televised conference before a meeting with the US envoy to Iraq, Putin said that, "The use of force abroad, according to existing international laws, can only be sanctioned by the United Nations. This is the international law. Everything that is done without the UN Security Council's sanction cannot be recognized as fair or justified."

US and UK officials have argued that the invasion was already authorized under existing UN Security Council resolutions regarding the 1991 Gulf War, the subsequent ceasefire (660, 678), and later inspections of Iraqi weapons programs (1441).

Critics of the invasion have challenged these assertions. They argued that an additional Security Council resolution, which the US and UK failed to obtain, would have been necessary to specifically authorize the invasion. In September 2004, then-United Nations Secretary-General Kofi Annan stated, "I have indicated that it is not in accordance with the UN charter. From our point of view and the UN Charter point of view, it [the war] was illegal."

The UN Security Council, as outlined in Article 39 of the UN Charter, has the ability to rule on the legality of the war. It has yet not been asked to do so by any UN member nation. Given that the United States and the United Kingdom have veto power in the Security Council, action by the Security Council is highly improbable even if the issue were to be raised. Despite this, the UN General Assembly (UNGA) may ask the International Court of Justice (ICJ)—"the principal judicial organ of the United Nations" (Article 92)—to give either an 'advisory opinion' or 'judgement' on the legality of the war.

Legitimacy

A dispute exists over the legitimacy of the 2003 invasion of Iraq. The debate centers around the question whether the invasion was an unprovoked assault on an independent country that may have breached international law, or if the United Nations Security Council authorized the invasion (whether the conditions set in place after the Gulf War allowed the resumption if Iraq did not uphold to the Security Council resolutions). Those arguing for its legitimacy often point to Congressional Joint Resolution 114 and UN Security Council resolutions, such as Resolution 1441 and Resolution 678."International Law and the War in Iraq," John Yoo. The American Journal of International Law, Vol. 97, No. 3 (Jul., 2003), pp. 563-576 ."Future Implications of the Iraq Conflict." W.H. Taft and T’F. Buchwald. The American Journal of International Law, Vol. 97, No. 3 (Jul., 2003), pp. 553-563 , Those arguing against its legitimacy also cite some of the same sources, stating they do not actually permit war but instead lay out conditions that must be met before war can be declared. Furthermore, the Security Council may only authorise the use of force against an "aggressor" in the interests of preserving peace, whereas the 2003 invasion of Iraq was not provoked by any aggressive military action.

There are ongoing debates regarding whether the invasion was launched with the explicit authorization of the United Nations Security Council. The Government of the United States asserts that the invasion was explicitly authorized by Security Council Resolution 678 and thus complies with international law. The Security Council Resolution 678 authorizes UN Member States to "use all necessary means to uphold and implement Resolution 660 and all subsequent relevant resolutions and to restore international peace and security in the area", however there exist different interpretations of its meaning. The only legal jurisdiction to find "aggression" or to find the invasion illegal rests with the Security Council under United Nations Charter Articles 39–42. The Security Council met in 2003 for two days, reviewed the legal claims involved, and elected to be "seized of the matter".Patrick McLaren, Settling the Score with Saddam: Resolution 1441 and Parallel Justifications for the Use of Force against Iraq (2003) 13 Duke Journal of Comparative and International Law 233 (Lexis)Bill Campbell and Chris Moraitis, 'Memorandum of Advice to the Commonwealth Government on the Use of Force against Iraq' (2003) 4 Melbourne Journal of International Law 178. The Security Council has not reviewed these issues since 2003. The public debate, however, continues. Former UN Secretary General Kofi Annan expressed his opinion that the invasion of Iraq was "not in conformity with the UN charter [...] from the charter point of view, [the invasion] was illegal".

Saddam's record While in power, Saddam Hussein invaded Iran in 1980 and began the Iran–Iraq War, which lasted until 1988. Iraq's invasion was backed by the United States who funneled over $5 billion to support Saddam's party and sold Iraq hundreds of millions of dollars' worth of military equipment. During the war, Hussein used chemical weapons on at least 10 occasions, including attacks against civilians. In 1990, Iraq invaded Kuwait and began the Persian Gulf War. After the ceasefire agreement was signed between Saddam and the UN in 1991, which suspended the hostilities of the Gulf War, Iraq repeatedly violated 16 different UNSC resolutions from 1990 to 2002. The Iraq Survey Group interviewed regime officials who stated Hussein kept weapon scientists employed and planned to revive Iraq's WMD program after the inspections were lifted, including nuclear weapons. Under UN Resolution 1441, he was given a "final opportunity" to comply, and he again violated the resolution by submitting a false report to UNMOVIC inspectors and continually preventing them from inspecting Iraq's WMD sites.

During the Gulf War, Iraq took foreign civilians hostage on an unprecedented scale. Hussein attempted to use terrorism against the United States during the Gulf War and against former President George H.W. Bush in Kuwait in 1993 for leading the Gulf War against him. He had a long history of supporting fighters in Palestine by giving money to families of suicide bombers and gave refuge to other fighting groups against neighboring states in the region.

In 2000, two human rights groups, International Federation of Human Rights Leagues and the Coalition for Justice in Iraq, released a joint report documenting the indoctrination of children into a fighting force. These children as young as five were recruited into the Ashbal Saddam or Saddam's Cubs. The children would be separated from their parents and undergo military training. Parents objecting to this recruitment would be executed and children jailed if they failed to comply. These jails were later noted by Scott Ritter in an interview with Time magazine.

Vice President Cheney stated in 2006 that the U.S. would still have invaded Iraq even if intelligence had shown that there were no weapons of mass destruction. He said Hussein was still dangerous because of his history of using WMD, and taking him out of power "was the right thing to do".

According to Donald Rumsfeld, Saddam Hussein was giving $25,000 to the families of suicide bombers who were aggressive toward Israel.

Weapons of mass destruction

In the past, Iraq had been supplied with chemical weapons and the technology to develop them by Germany, France, United States and the United Kingdom. Saddam used these weapons against Iranian forces in the Iran–Iraq War, and against Kurdish civilians in the Iraqi town of Halabja. In 1990 during the Gulf War Saddam had the opportunity to use these weapons, but chose not to. One of the noted reasons is the Iraqi forces' lack of up to date equipment to protect themselves from the effects, as well as the speed with which the US forces traversed the open desert. From 1991 to 1998 UNSCOM inspected Iraq and worked to locate and destroy WMD stockpiles. The team was replaced in 1999 with the United Nations Monitoring Verification and Inspection Commission, UNMOVIC.

In 2002, Scott Ritter, a former UNSCOM weapons inspector, heavily criticized the Bush administration and the news media for relying on the testimony of alleged Iraqi nuclear scientist and defector Khidir Hamza as a rationale for invading Iraq.

No militarily significant WMDs have been found in Iraq since the invasion, although several degraded chemical munitions dating to before 1991 have been. On 21 June 2006, a report was released through the United States Senate Select Committee on Intelligence, stating that since 2003, approximately 500 degraded chemical munitions have been discovered dating from before 1991 in Iraq, and "likely more will be recovered". The weapons are filled "most likely" with Sarin and Mustard Gas. However, the U.S. Department of Defense states that these weapons were not in usable condition, and that "these are not the WMDs this country and the rest of the world believed Iraq had, and not the WMDs for which this country went to war".

In January 2006, The New York Times reported that, "A high-level intelligence assessment by the Bush administration concluded in early 2002 that the sale of uranium from Niger to Iraq was 'unlikely'." The Iraqi government denied the existence of any such facilities or capabilities and called the reports lies and fabrications, which was backed by the post-war prima facie case that no WMDs were evident or found.

Former CIA officials have stated that the White House knew before the invasion that Iraq had no weapons of mass destruction, but had decided to attack Iraq and continue to use the WMD story as a false pretext for launching the war. The leaked Downing Street Memo, an internal summary of a meeting between British defense and intelligence officials, states that Bush administration had decided to attack Iraq and to "fix intelligence" to support the WMD pretext to justify it. A transcript of a secret conversation between President Bush and PM Blair leaked by a government whistleblower reveals that the US and UK were prepared to invade Iraq even if no WMD were found. (see also the Downing Street Memorandum) British officials in the memo also discuss a proposal by President Bush to provoke Iraq, including using fake UN planes, to manufacture a pretext for the invasion he had already decided on.

In 2004 the Butler Commission Report concluded that, "on the basis of the intelligence assessments at the time", statements by the British Government "on Iraqi attempts to buy uranium from Africa" were "well-founded". Opponents however consider the Butler Review a whitewash which lacked cross-party support (the panel was appointed by, and reported directly to, the Prime Minister).

The Iraq Intelligence Commission rejected claims that the Bush administration attempted to influence the intelligence community's pre-war assessments of Iraq's weapons programs, but it did not investigate whether the administration misled the public about the intelligence.

Paul R. Pillar, a 28-year veteran of the CIA, wrote in Foreign Affairs that the method of investigation used by [these] panels—essentially, asking analysts whether their arms had been twisted—would have caught only the crudest attempts at politicization: