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 the Iranian Revolution. Although 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 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 came at a great cost in lives and economic damage - a half a million Iraqi and Iranian soldiers as well as civilians are believed to have died in the war with many more injured and wounded - but brought neither reparations nor change in borders. The conflict is often compared to World War I, in that the tactics used closely mirrored those of the 1914-1918 war, including large scale trench warfare, manned machine-gun posts, bayonet charges, use of barbed wire across trenches and on no-mans land, 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. At the time, the UN Security Council issued statements that "chemical weapons had been used in the war." However, in these UN statements Iraq was not mentioned by name, so it has been said that "the international community remained silent as Iraq used weapons of mass destruction against Iranian as well as Iraqi Kurds" and it is believed that United States prevented the UN from condemning Iraq