The Iran–Iraq War (Persian: جنگ ایران و عراق; Arabic: الحرب الإيرانية العراقية) was a protracted armed conflict that began on 22 September 1980 with a full-scale invasion of Iran by neighbouring Iraq. The war lasted for almost eight years, and ended in a stalemate on 20 August 1988, when Iran accepted Resolution 598 of the United Nations Security Council. Iraq's primary rationale for the invasion was to cripple Iran and prevent Ruhollah Khomeini from exporting the 1979 Iranian Revolution movement to Shia-majority Iraq and internally exploit religious tensions that would threaten the Sunni-dominated Ba'athist leadership led by Saddam Hussein. Iraq also wished to replace Iran as the dominant state in the Persian Gulf, which, prior to the Iranian Revolution, was not seen as an achievable objective by the Iraqi leadership due to pre-revolutionary Iran's colossal economic and military power as well as its close alliances with the United States, a superpower, and Israel, a major player in the Middle East. The war followed a long-running history of bilateral border disputes between the two states, as a result of which Iraq planned to retake the eastern bank of the Shatt al-Arab ceded in 1975. Iraq supported Arab separatists in the oil-rich territory of Khuzestan seeking an Arab state known as "Arabistan" who had started an insurgency in 1979 with support from Iraq. Iraq sought to seize control of and sever Khuzestan from Iran. Saddam Hussein publicly stated in November 1980 that Iraq was not seeking annexation of Khuzestan into Iraq; rather, it is believed that Iraq sought to establish a suzerainty over the territory.While the Iraqi leadership had hoped to take advantage of Iran's post-revolutionary chaos and expected a decisive victory in the face of a severely weakened Iran, the Iraqi military only made progress for three months, and by December 1980, the Iraqi invasion of Iran had stalled. As fierce fighting broke out between the two sides, the Iranian military began to gain momentum against the Iraqis and regained virtually all of its lost territory by June 1982. After pushing Iraqi forces back to the pre-war border lines, Iran invaded Iraq and went on the offensive for the next five years until the latter took back the initiative in mid-1988 and launched a series of major counter-offensives that ultimately led to the conclusion of the war in a stalemate. There were a number of proxy forces operating for both countries—most notably the People's Mujahedin of Iran, which had sided with Iraq, and the Iraqi Kurdish militias of the KDP and PUK, which had sided with Iran. The United States, United Kingdom, Soviet Union, France, and many Arab countries provided an abundance of financial, political and logistical support for Iraq. While Iran was comparatively isolated to a large degree, it received various forms of support, with its most notable sources of aid being Syria, Libya, China, North Korea, Israel, Pakistan and South Yemen.

The eight years of war-exhaustion, economic devastation, decreased morale, military stalemate, inaction by the international community towards the use of weapons of mass destruction by Iraqi forces on Iranian civilians as well as increasing U.S.–Iran military tensions all culminated in Iran's acceptance of a ceasefire brokered by the United Nations.

The conflict has been compared to World War I in terms of the tactics used, including large-scale trench warfare with barbed wire stretched across fortified defensive lines, manned machine gun posts, bayonet charges, Iranian human wave attacks, extensive use of chemical weapons by Iraq, and deliberate attacks on civilian targets. A notable feature of the war was the state-sanctioned glorification of martyrdom to Iranian children, which had been developed in the years before the revolution. The discourses on martyrdom formulated in the Iranian Shia Islamic context led to the tactics of "human wave attacks" and thus had a lasting impact on the dynamics of the war.In total, around 500,000 people were killed during the war (with Iran bearing the larger share of the casualties), excluding the tens of thousands of civilians killed in the concurrent Anfal campaign targeting Kurds in Iraq. The end of the war resulted in neither reparations nor border changes. The combined financial cost to both combatants is believed to have exceeded US$1 trillion.