The Iran–Iraq War was an armed conflict between Iran and Iraq that lasted from September 1980 to August 1988. It began with the Iraqi invasion of Iran and lasted for eight years, until the acceptance of United Nations Security Council Resolution 598 by both sides. Iraq's primary rationale for the attack against Iran cited the need to prevent Ruhollah Khomeini—who had spearheaded the Iranian Revolution in 1979—from exporting the new Iranian ideology to Iraq; there were also fears among the Iraqi leadership of Saddam Hussein that Iran, a theocratic state with a population predominantly composed of Shia Muslims, would exploit sectarian tensions in Iraq by rallying Iraq's Shia majority against the Baʽathist government, which was officially secular and dominated by Sunni Muslims. Iraq also wished to replace Iran as the power player in the Persian Gulf, which was not seen as an achievable objective prior to the Islamic Revolution because of Pahlavi Iran's economic and military superiority as well as its close relationships with the United States and Israel.

The Iran–Iraq War followed a long-running history of territorial border disputes between the two states, as a result of which Iraq planned to retake the eastern bank of the Shatt al-Arab that it had ceded to Iran in the 1975 Algiers Agreement. Iraqi support for Arab separatists in Iran increased following the outbreak of hostilities; it is widely accepted among scholars that Iraq was seeking to annex, or at least to establish suzerainty over, Iran's Khuzestan province, but Saddam Hussein publicly denied this in November 1980. 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 had stalled. The Iranian military began to gain momentum against the Iraqis and regained virtually all lost territory by June 1982. After pushing Iraqi forces back to the pre-war border lines, Iran rejected United Nations Security Council Resolution 514 and launched an invasion of Iraq. The subsequent Iranian offensive within Iraqi territory lasted for five years, with Iraq taking back the initiative in mid-1988 and subsequently launching a series of major counter-offensives that ultimately led to the conclusion of the war in a stalemate.

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 soldiers and civilians, as well as increasing Iran–United States military tensions all culminated in Iran's acceptance of a ceasefire brokered by the United Nations Security Council. In total, around 500,000 people were killed during the Iran–Iraq War (with Iran bearing the larger share of the casualties), excluding the tens of thousands of civilians killed in the concurrent Anfal campaign that targeted Iraqi Kurdistan. The end of the conflict resulted in neither reparations nor border changes, and the combined financial losses suffered by both combatants is believed to have exceeded US$1 trillion. There were a number of proxy forces operating for both countries: Iraq and the pro-Iraqi Arab separatist militias in Iran were most notably supported by the National Council of Resistance of Iran; whereas Iran re-established an alliance with the Iraqi Kurds, being primarily supported by the Kurdistan Democratic Party and the Patriotic Union of Kurdistan. During the conflict, Iraq received an abundance of financial, political, and logistical aid from the United States, the United Kingdom, the Soviet Union, France, Italy, Yugoslavia, and the overwhelming majority of Arab countries. While Iran was comparatively isolated to a large degree, it received a significant amount of aid from Syria, Libya, China, North Korea, Israel, Pakistan, and South Yemen.

The conflict has been compared to World War I in terms of the tactics used by both sides, including large-scale trench warfare with barbed wire stretched across fortified defensive lines, manned machine-gun posts, bayonet charges, Iranian human wave attacks, Iraq's extensive use of chemical weapons, and deliberate attacks on civilian targets. A notable feature of the war was the state-sanctioned glorification of martyrdom among Iranian children; the discourses on martyrdom formulated in the Iranian Shia Islamic context led to the widespread usage of human wave attacks and thus had a lasting impact on the dynamics of the conflict.