The Iran-Iraq War, also known as the Imposed War by Iraq (جنگ تحمیلی, Jang-e-tahmīlī), Iranian Holy Defense (دفاع مقدس, Defa-e-moghaddas) and Iranian Revolutionary War in Iran, and Saddām's Qādisiyyah (قادسيّة صدّام, Qādisiyyat Saddām) in Iraq, was a war between the armed forces of Iraq and Iran lasting from September 1980 to August 1988. It was commonly referred to as the Persian Gulf War until the Iraq-Kuwait conflict of (1990–91), and for a while thereafter as the First Persian Gulf War. The Iraq-Kuwait conflict, while originally known as the Second Persian Gulf War, later became known simply as the Persian Gulf War.

The war began when Iraq invaded Iran on 22 September 1980 following a long history of border disputes and fears of Shia insurgency among Iraq's long suppressed Shia majority influenced by Iran's Islamic revolution. Although Saddam's Iraq hoped to take advantage of revolutionary chaos in Iran and attacked without formal warning, they made only limited progress into Iran and within several months were repelled by the Iranians who regained virtually all lost territory by June 1982. For the next six years Iran was on the offensive Despite several calls for a ceasefire by the United Nations Security Council, hostilities continued until 20 August 1988. The last prisoners of war were exchanged in 2003.

The war is noted for being very similar to World War I. Tactics used included trench warfare, manned machine-gun posts, bayonet charges, use of barbed wire, human wave attacks and Iraq's extensive use of chemical weapons (such as mustard gas) against Iranian troops and civilians as well as Iraqi Kurds.