The Iran-Iraq War

Shortly after the 1979 revolution, a war broke out between Iran and Iraq that would engulf both nations for a decade and leave lasting scars.

According to Wikipedia, this war, which lasted from 1980 to 1988, was the longest conventional war of the twentieth century. Iran had been a regional rival with Iraq for centuries, and tensions intensified in the modern era when the two nations became competitive over oil resources. 

Saddam Hussein, who became president of Iraq in 1979, the same year that the revolution in Iran swept Ayatollah Khomeini to power, saw the political chaos generated by the revolution as a prime opportunity to capture Iran's territory and oil fields. He also feared that Shia Muslims in his own country, who made up a majority of the population but were held back from political influence by the Sunni Muslims in Saddam Hussein's Baath party, would be encouraged by the revolution.

Iraq invaded Iran in September 1980. The invasion was successfully resisted, and the Iranians retaliated. By 1982, the major front of the war was no longer in Iran's territory, but in Iraq's. The Iraqis hunkered down in trenches. The resulting drawn-out trench warfare, with neither side advancing or retreating, was reminiscent of conditions in World War I, but with more advanced and devastating weaponry.

The Iranians used human wave attacks spearheaded by the Basij paramilitary units, in which large amounts of soldiers charge an enemy's defensive position. This resulted in heavy casualties for the Iranians. Saddam Hussein's army used chemical weapons and deadly gas against the Iranian enemy, as well as against the Kurds in Iraq during this war. When the ground war became a stalemate, both sides launched missiles at each other's cities, though Iraq did more damage due to its superior air power.

The U.S. supported Iraq during this conflict, since its relationship with Iran had deteriorated as a result of the revolution and year-long hostage crisis. The photo of Donald Rumsfeld shaking hands with Saddam Hussein, which later became infamous when Rumsfeld invaded Iraq as Secretary of Defense in 2003, shows him pledging support to Saddam's government during this period.

When the war ended in 1988, Iran had taken about a million casualties and Iraq about 500,000. Relations between the two countries remain bitter, even after the fall of Saddam's government. U.S.-Iran relations are not much better, and Iran now supports groups who fight against the U.S. and its allies in the Middle East, including Hezbollah in Lebanon and Shiite militias in Iraq.