The US invasion of Iraq 2003 (March 20 – May 1, 2003), was the start of the conflict known as the Iraq War or Operation Iraqi Freedom in which a combined force of troops from the United States, alongside the United Kingdom, and smaller contingents from Australia and Poland invaded Iraq and toppled the regime of Saddam Hussein in 21 days of major combat operations. This phase (March–April 2003) consisted of a conventionally fought war which concluded with the fall of Baghdad that marked the beginning of the second phase, the Iraq War which would last until August 31, 2010, and was followed by operation new dawn. This was considered a continuation of the Gulf War of 1991, prior to which Saddam Hussein had invaded Kuwait, and after defeat by Coalition Forces had agreed to surrender and/or destroy several types of weapons, including SCUD missiles and weapons of mass destruction (WMD).

Since the Persian Gulf War 1991 the US and Britain had been keeping a tight rein on Saddam Hussein, waging an undeclared conflict against Iraq for twelve years. U.S. President Bill Clinton had maintained sanctions and ordered air strikes in the "Iraqi no-fly zones" Operation Desert Fox, in the hope that Saddam would be overthrown by political enemies inside Iraq and had signed into law H.R. 4655, the Iraq Liberation Act. which appropriated funds to Iraqi opposition groups.Four countries participated with troops during the initial invasion phase, which lasted from March 20 to May 1, 2003. These were the United States (248,000), United Kingdom (45,000), Australia (2,000), and Poland (194). 36 other countries were involved in its aftermath. In preparation for the invasion, 100,000 US troops were assembled in Kuwait by February 18. The United States supplied the vast majority of the invading forces, but also received support from Kurdish irregulars in Iraqi Kurdistan.

According to U.S. President George W. Bush and British Prime Minister Tony Blair, the reasons for the invasion were "to disarm Iraq of weapons of mass destruction, to end Saddam Hussein's alleged support for terrorism, and to free the Iraqi people." According to Blair, the trigger was Iraq's failure to take a "final opportunity" to disarm itself of nuclear, chemical, and biological weapons that US and British officials called an immediate and intolerable threat to world peace. Although some remnants of pre-1991 production were found after the end of the war, US government spokespeople confirmed that these were not the weapons for which the US went to war. There also have been claims that the war was waged in order to take oil from Iraq. In 2005, the Central Intelligence Agency released a report saying that no weapons of mass destruction had been found in Iraq.

There was increasingly strong pressure among American policy influencers, from the mid-1990s on, that regime change in Iraq was important to the overall goals of American foreign policy. In a January 2003 CBS poll 64% of US nationals had approved of military action against Iraq, however 63% wanted Bush to find a diplomatic solution rather than go to war, and 62% believed the threat of terrorism directed against the US would increase due to war. The invasion of Iraq was strongly opposed by some traditional US allies, including the governments of France, Germany, New Zealand, and Canada. Their leaders argued that there was no evidence of weapons of mass destruction in Iraq and that invading the country was not justified in the context of UNMOVIC's February 12, 2003 report. On February 15, 2003, a month before the invasion, there were worldwide protests against the Iraq war, including a rally of three million people in Rome, which is listed in the Guinness Book of Records as the largest ever anti-war rally.^[28] According to the French academic Dominique Reynié, between January 3 and April 12, 2003, 36 million people across the globe took part in almost 3,000 protests against the Iraq war.

The invasion was preceded by an air strike on the Presidential Palace in Baghdad on March 19, 2003. The following day coalition forces launched an incursion into Basra Province from their massing point near the Iraqi-Kuwaiti border. While commandos launched an amphibious assault from the Persian Gulf to secure Basra and the surrounding petroleum fields, the main invasion army moved into southern Iraq, occupying the region and engaging in the Battle of Nasiriyah on March 23. Massive air strikes across the country and against Iraqi command and control threw the defending army into chaos and prevented an effective resistance.

On March 26 the 173rd Airborne Brigade was airdropped near the northern city of Kirkuk where they joined forces with Kurdish rebels and fought several actions against the Iraqi army to secure the northern part of the country.

The main body of coalition forces continued their drive into the heart of Iraq and met with little resistance. Most of the Iraqi military was quickly defeated and Baghdad was occupied on April 9. Other operations occurred against pockets of the Iraqi army including the capture and occupation of Kirkuk on April 10, and the attack and capture of Tikrit on April 15. Iraqi President Saddam Hussein and the central leadership went into hiding as the coalition forces completed the occupation of the country. On May 1 an end of major combat operations was declared, ending the invasion period and beginning the military occupation period.