The tanker war, part of the larger Iran–Iraq War, was a series of military attacks by Iran and Iraq against merchant vessels in the Persian Gulf and Strait of Hormuz from 1981 to 1988. Iraq was responsible for 283 attacks while Iran accounted for 168.

The Iran-Iraq war began in September 1980. Before 1984, attacks against shipping had occurred, albeit on a much smaller scale. In December 1980, UN Secretary General Kurt Waldheim appealed to Iran and Iraq to ensure the security of peaceful shipping in the Persian Gulf. At that time, Iran assured other countries that it would keep the Strait of Hormuz open.

In his 1988 review paper naval affairs analyst Ronald O'Rouke wrote that "Iran trapped or destroyed many Iraqi ships in port in the early stages of the war. But Iraq started the tanker war in the Gulf proper in 1981 by initiating attacks on ships steaming to or from Iranian ports at the extreme northern end of the Gulf. Iraq continued these attacks into 1984 without a parallel Iranian response at sea. In March of that year, however, Iraq increased the rate of its attacks and expanded their geographic scope by attacking ships serving more southerly Iranian points, particularly the oil-loading complex at Kharg Island. Two months later, Iran initiated its own attacks, and the tanker war became a two-way affair."

Iraq broadened the tanker war in 1984 by attacking the oil terminal and oil tankers at Kharg Island. Iraq's aim in attacking Iranian shipping was to provoke the Iranians to retaliate with extreme measures, such as closing the Strait of Hormuz to all maritime traffic, thereby bringing about foreign intervention against Iran; the United States had threatened several times to intervene if the Strait of Hormuz were closed. Kuwait and Saudi Arabia supported Iraq against Iran. The United States intervened in the conflict in 1986 to protect Kuwaiti tankers, and engaged in a confrontation with Iran.

Both sides had declared an "exclusion zone", meaning areas in which they had warned ships from entering. Iraq declared the area around Iran's Kharg Island to be an exclusion zone. Kharg Island hosted Iran's principal oil shipment port. Iraq gave precise definition, in coordinates, of this exclusion zone and gave advance notification to all countries. However, Iraq did not designate any safe passage routes in this zone.

Iran declared all waters within 40 miles of its coast to be its exclusion zone. It instructed ships headed for non-Iranian ports to sail west of this line. While Iran also did not designate any safe passages in its exclusion zone, this was unnecessary. Iran's exclusion zone allowed for ships to enter and exit the Gulf, and essentially only kept such foreign ships out of its own waters.

Iran's exclusion zone made it easier for Iraq to target Iranian ships. It allowed Iraq to assume that any ship in Iran's territorial waters must be going to (or coming from) an Iranian port.

War studies scholar Stephen Phillips writes that "In January 1987, the Kuwaiti government proposed a clever scheme to deter Iranian attacks against their shipping. They asked the United States if they could reflag Kuwaiti tankers as American and receive the protection of the U.S. Navy. The administration of President Ronald Reagan debated this idea but finally agreed to it on March 7, 1987." These are known to O'Rourke as "reflagged Kuwaiti ships".