Iran-Iraq War
Fought between September 1980 and August 1988, with prisoners returned as late as 2003, the Iran-Iraq War was one of the longer, and certainly bloodier, of modern wars. Much of the conflict was a static stalemate reminiscent of the trench warfare of the First World War. Iraq, which had invaded Iran over a territorial dispute, suddenly asked for peace in 1988, in what may well have been a plan by Saddam Hussein to shift his attention to became the 1990 invasion of Kuwait, starting the Gulf War. Several other disputes, including Islamic sectarian conflict and freedom of navigation in the Persian Gulf, led to parallel fighting.
A substantial number of countries supplied and advised Iran, Iraq, or, in a number of cases, both, although there were no significant units from other countries directly involved in the fighting between Iran and Iraq. Starting in 1984, there was fighting between Iran and the U.S. and some of its allies, but that was more of a parallel conflict, often called the Tanker War.
Many countries in the West had regarded Iran, and to a lesser extent Iraq, as a buffer against Soviet expansion. The rich oil reserves of both countries also was important to the major powers. These priorities had created some important background for regional tensions, such as Operation Ajax, a U.S. and British covert operation that deposed the popular, if left-leaning, government of Mohammed Mossadegh, and restored the Pahlavi Dynasty. Shah Mohammed Reza Pahlavi was aggressively Westernizing Iran, offending a good deal of conservative Shi'ite Muslim opinion, and governing with the assistance of an aggressive secret police apparatus.
Contributing to the attitudes of other countries was the Iranian Islamic Revolution that deposed the Shah. In reaction to the U.S. giving sanctuary and hospitalization to the Shah, members of the loosely organized revolutionary forces. This led to the 1979 Iranian seizure of the U.S. Embassy, with diplomats held hostage until Ronald Reagan took office. The seizure caused intense friction between the U.S. and Iran.
Background
Iraq invades Iran
Saddam Hussein, ostensibly a Sunni Muslim, claimed to have started the war to block the export of Shi'ite radical Islam by Ayatollah Ruhollah Khomeini, and also to gain control of the Shatt al-Arab, a relatively narrow body of water that provides sea access to Iraq's main port of Basra. While Iraq, at first, pushed back Iranian troops after the September 22, 1980 invasion. By March 1981, however, the Iraqi momentum had stalled.
After the situation was stalemated, there were various attempts to broker a settlement. Saudi Arabia offered to pay reparations to Iran and guarantee the removal of Iraqi troops, but Khomeini rejected that proposal.
Iranian counteroffensive
On 13 July, Iran opened a major counteroffensive.
Offensives and counteroffensives
Between January 1985 and February 1986, both sides launched offensives, all of which ground to a halt.
Tanker War
Iran and Iraq both saw oil revenue as affecting the other side's ability to fund its war, and began acting to deny such revenues. This involved attacks on third-country shipping and on ports containing third-country ships.
Essentially independently of the main fighting, Iran was increasingly involved with the U.S. and allies such as Kuwait. The period was rife with bad judgment, such as the U.S. shooting down a misidentified Iranian civilian airliner, Iran Air flight 655 and the Iraqis attacking a patrolling U.S. frigate, U.S.S. Stark
War of the Cities
Each side then tried to break the morale of the others by attacks on cities, primarily by wildly inaccurate SCUD ballistic missiles and derivatives, but also with small raids by aircraft. As in WWII, this caused misery but was indecisive.