The Iraq War, also known as the Second Gulf War, Operation Iraqi Freedom (US), Operation TELIC (UK) or the occupation of Iraq, is an ongoing conflict which began on March 20, 2003 with the United States-led invasion of Iraq by a multinational coalition composed of U.S. and U.K. troops supported by smaller contingents from Australia, Poland, and other nations.

The main rationale for the invasion offered by U.S. President George W. Bush and coalition supporters was the allegation that Iraq possessed and was actively developing weapons of mass destruction (WMD) in violation of a 1991 agreement. U.S. officials argued that Iraq posed an imminent, urgent, and immediate threat to the United States, its people, allies, and interests. The supporting intelligence was widely criticized, and weapons inspectors found no evidence of WMD. After the invasion, the Iraq Survey Group concluded that Iraq had ended its WMD programs in 1991 and had none at the time of the invasion, but that they intended to resume production if and when the Iraq sanctions were lifted. Although some earlier degraded remnants of misplaced or abandoned WMD were found, they were not the weapons for which the coalition invaded. Some U.S. officials claimed Saddam Hussein and al-Qaeda had been cooperating, but no evidence of any collaborative relationship has been found. Other reasons for the invasion stated by officials included concerns over Iraq's financial support for the families of Palestinian suicide bombers, Iraqi government human rights abuses, spreading democracy, and Iraq's oil reserves.

The invasion led to the quick defeat of the Iraqi military, the flight of President Saddam Hussein, his capture in December, 2003 and his execution in December, 2006. The U.S.-led coalition occupied Iraq and attempted to establish a new democratic government. But shortly after the initial invasion, violence against coalition forces and among various sectarian groups led to asymmetric warfare with the Iraqi insurgency, civil war between many Sunni and Shia Iraqis, and al-Qaeda operations in Iraq. Estimates of the number of people killed range from over 150,000 to more than 1 million. The financial cost of the war has been more than £4.5 billion ($9 billion) to the UK, and over $845 billion to the U.S., with the total cost to the U.S. economy estimated at $3 to 5 trillion. Member nations of the Coalition began to withdraw their forces as public opinion favoring troop withdrawals increased and as Iraqi forces began to take responsibility for security. Because of several similarities, namely that the war has dragged on without success and with continuing casualties inflicted by insurgents, the war has often been referred to as another Vietnam War for the United States.