Although Zarqawi’s rhetoric was similar to bin Laden’s, his targets were quite different. From the start, Zarqawi directed his malevolence at fellow Muslims, especially Iraq’s majority Shiite population. Bin Laden and al-Qaeda regarded the Shiites as heretics, but rarely targeted them for slaughter.

Zarqawi’s intentions were underlined with the bombing of the Imam Ali shrine in Najaf, the holiest place of Shiite worship in Iraq. I was at the shrine when it happened, and remember many survivors asking, “Why us? Why, when there are so many Americans around, bomb us?”

One reason: sheer convenience. The Shiites were easier targets because they didn’t yet have the ability to fight back. But there was also a political calculation. After Saddam was toppled, Shiite politicians replaced the Sunnis who had long dominated power structures in Iraq. Zarqawi was counting on Sunni resentment against the Shiites to build alliances and find safe haven for his group. It worked: Zarqawi sent dozens of suicide bombers to blow themselves up in mosques, schools, cafes, and markets, usually in predominantly Shiite neighborhoods or towns.

AQI: The rise and fall

By 2004, Zarqawi’s campaign of suicide bombings across Iraq had made him a superstar of the international “jihadi” movement, and won the endorsement of bin Laden himself. Zarqawi now joined his group to bin Laden’s, rebranding it al-Qaeda in Iraq, or AQI. (It is also sometimes called al-Qaeda in Mesopotamia, but don’t confuse that with AQIM, which refers to the Algerian franchise, al-Qaeda in the Maghreb.)

Soon, however, Zarqawi’s targeting of civilians created misgivings among the core al-Qaeda leadership. In 2005, bin Laden’s right-hand man, Ayman al-Zawahiri, wrote a letter chiding the Jordanian for his tactics. Zarqawi paid it no heed. Last year Zawahiri likewise took ISIS’s new leader, Abu Bakr al-Baghdadi, to task for his excessive ferocity—and was again ignored.

By the spring of 2006, Zarqawi was beginning to see himself as something more than an “emir” or insurgent commander: He aspired to spiritual leadership as well. (His successor as “emir,” Baghdadi, would make the same transformation, appointing himself “caliph” after taking Mosul.) No longer content merely with alliances, he began to insist that his Iraqi Sunni hosts submit to his harsh interpretation of sharia law—veils for women, beheadings for criminals, the whole nine yards. Those who resisted, even prominent figures in the community, were executed.

But Zarqawi’s ambitions were cut short in June, 2006, when the U.S. Air Force dropped a pair of 500-pound bombs on his hideout, 20 miles north of Baghdad.

His death came just as the tide was turning against AQI. Many Sunni tribes, chafing at Zarqawi’s sharia rules, had begun to fight back. The U.S. military, led by General David Petraeus, capitalized on this to finance and support an insurgency-within-an-insurgency, known as the “Awakening.” Tribesmen willing to fight AQI, even if they had previously fought the Americans, were designated “Sons of Iraq,” to underscore the fact that most of AQI’s commanders were foreigners, like Zarqawi himself. These Iraqi Sunnis believed that joining forces with the U.S. would give them immunity from prosecution from previous crimes, lucrative government contracts to rebuild devastated Sunni areas, and a share of political power in Baghdad.