Countdown Time fo Iraq War: While there had been some earlier talk of action against Iraq, the Bush administration waited until September 2002 to call for action, with White House Chief of Staff Andrew Card saying, "From a marketing point of view, you don't introduce new products in August." Bush began formally making his case to the international community for an invasion of Iraq in his September 12, 2002 address to the UN Security Council. Key US allies in NATO, such as the United Kingdom, agreed with the US actions, while France and Germany were critical of plans to invade Iraq, arguing instead for continued diplomacy and weapons inspections. After considerable debate, the UN Security Council adopted a compromise resolution, UN Security Council Resolution 1441, which authorized the resumption of weapons inspections and promised "serious consequences" for noncompliance. Security Council members France and Russia made clear that they did not consider these consequences to include the use of force to overthrow the Iraqi government. Both the US ambassador to the UN, John Negroponte, and the UK ambassador, Jeremy Greenstock, publicly confirmed this reading of the resolution, assuring that Resolution 1441 provided no "automaticity" or "hidden triggers" for an invasion without further consultation of the Security Council.

Resolution 1441 gave Iraq "a final opportunity to comply with its disarmament obligations" and set up inspections by the United Nations Monitoring, Verification and Inspection Commission (UNMOVIC) and the International Atomic Energy Agency (IAEA). Hussein accepted the resolution on November 13 and inspectors returned to Iraq under the direction of UNMOVIC chairman Hans Blix and IAEA Director General Mohamed ElBaradei. As of February 2003, the IAEA "found no evidence or plausible indication of the revival of a nuclear weapons program in Iraq"; the IAEA concluded that certain items which could have been used in nuclear enrichment centrifuges, such as aluminum tubes, were in fact intended for other uses. UNMOVIC "did not find evidence of the continuation or resumption of programs of weapons of mass destruction" or significant quantities of proscribed items. UNMOVIC did supervise the destruction of a small number of empty chemical rocket warheads, 50 liters of mustard gas that had been declared by Iraq and sealed by UNSCOM in 1998, and laboratory quantities of a mustard gas precursor, along with about 50 Al-Samoud missiles of a design that Iraq claimed did not exceed the permitted 150 km range, but which had travelled up to 183 km in tests. Shortly before the invasion, UNMOVIC stated that it would take "months" to verify Iraqi compliance with resolution 1441.

In October 2002 the US Congress passed a "Joint Resolution to Authorize the Use of United States Armed Forces Against Iraq". The resolution authorized the President to "use any means necessary" against Iraq, Americans polled in January 2003 widely favored further diplomacy over an invasion. Later that year, however, Americans began to agree with Bush's plan. The US government engaged in an elaborate domestic public relations campaign to market the war to its citizens. Americans overwhelmingly believed Hussein did have weapons of mass destruction: 85% said so, even though the inspectors had not uncovered those weapons. Of those who thought Iraq had weapons sequestered somewhere, about half responded that said weapons would not be found in combat. By February 2003, 74% of Americans supported taking military action to remove Hussein from power.

The Central Intelligence Agency's Special Activities Division (SAD) teams were the first US forces to enter Iraq, in July 2002, before the main invasion. Once on the ground, they prepared for the subsequent arrival of US Army Special Forces to organize the Kurdish Peshmerga. This joint team (called the Northern Iraq Liaison Element (NILE)) combined to defeat Ansar al-Islam, a group with ties to al Qaeda, in Iraqi Kurdistan. This battle was for control of the territory that was occupied by Ansar al-Islam and took place before the invasion. It was carried out by Paramilitary Operations Officers from SAD and the Army's 10th Special Forces Group. This battle resulted in the defeat of Ansar and the capture of a chemical weapons facility at Sargat. Sargat was the only facility of its type discovered in the Iraq war.

SAD teams also conducted missions behind enemy lines to identify leadership targets. These missions led to the initial air strikes against Hussein and his generals. Although the strike against Hussein was unsuccessful in killing him, it effectively ended his ability to command and control his forces. Strikes against Iraq's generals were more successful and significantly degraded the Iraqi command's ability to react to, and maneuver against the US-led invasion force. SAD operations officers were also successful in convincing key Iraqi Army officers into surrendering their units once the fighting started.

NATO member Turkey refused to allow the US army across its territory into northern Iraq. Therefore, joint SAD and Army Special forces teams and the Pershmerga were the entire Northern force against the Iraqi army. They managed to keep the northern divisions in place rather than allowing them to aid their colleagues against the US led coalition force coming from the south. Four of these CIA officers were awarded the Intelligence Star for their actions.

In February 2003, US Secretary of State Colin Powell addressed the United Nations General Assembly, continuing US efforts to gain UN authorization for an invasion. Powell presented evidence alleging that Iraq was actively producing chemical and biological weapons and had ties to al-Qaeda. As a follow-up to Powell’s presentation, the United States, United Kingdom, Poland, Italy, Australia, Denmark, Japan, and Spain proposed a resolution authorizing the use of force in Iraq, but NATO members like Canada, France, and Germany, together with Russia, strongly urged continued diplomacy. Facing a losing vote as well as a likely veto from France and Russia, the US, UK, Spain, Poland, Denmark, Italy, Japan, and Australia eventually withdrew their resolution.

The decision to invade was widely unpopular worldwide, and opposition to the invasion coalesced on February 15 in a worldwide anti-war protest that attracted between six and ten million people in more than 800 cities, the largest such protest in human history according to the Guinness Book of World Records.

In March 2003, the United States, United Kingdom, Spain, Australia, Poland, Denmark, and Italy began preparing for the invasion of Iraq, with a host of public relations and military moves. In his March 17, 2003 address to the nation, Bush demanded that Hussein and his two sons, Uday and Qusay, surrender and leave Iraq, giving them a 48-hour deadline. But the US began the bombing of Iraq on the day before the deadline expired. On March 18, 2003, the bombing of Iraq by the United States, the United Kingdom, Spain, Italy, Poland, Australia, and Denmark began. Unlike the first Gulf War or the war in Afghanistan (2001–present), this war had no explicit UN authorisation.

The UK Parliament held a debate on going to war on March 18, 2003 where the government motion was approved 412 to 149. The vote was a key moment in the history of the Blair administration, as the number government MPs that rebelled against the vote was the greatest since the repeal of the Corn Laws. Three government ministers resigned in protest at the war, John Denham, Lord Hunt of Kings Heath, and the then Leader of the House of Commons Robin Cook. Speaking in the House of Commons after his resignation he made a passionate speech. He said, "What has come to trouble me is the suspicion that if the 'hanging chads' of Florida had gone the other way and Al Gore had been elected, we would not now be about to commit British troops to action in Iraq." During the debate it was stated that the Attorney General had advised that the war was legal under previous UN Resolutions.