Illustration by Edel Rodriguez for FP

The new Iraq was supposed to be a model for the Middle East and a threat to Iran’s theocracy. Instead, Iran has emerged as the biggest winner of the United States’ war. There is little stability or democracy in Iraq to impress Iranians. Conjuring more fear than hope, the war did nothing to loosen the grip of clerical rule over the country. Iranians rejoiced in the fall of Saddam, who fought an eight-year war against their country that killed hundreds of thousands of people, many by chemical weapons. For Iran, the war in Iraq turned out to be a strategic windfall, uprooting Baathism and pacifying a nemesis that had been a thorn in its side for much of the 20th century. Iraq’s new Shiite—and in good measure, Kurdish—masters enjoy friendly ties with Iran. It was no coincidence that Iran was the first of Iraq’s neighbors to recognize its new government and to encourage Iraqis to participate in the political process introduced by the United States.

In the political vacuum that followed Saddam’s fall, Iranian influence quickly spread into southern Iraq on the back of commercial connections—driven by a growing volume of trade and a massive flow of Iranian pilgrims into shrine cities of Iraq—and burgeoning intelligence and political ties. Iran’s influence quickly extended to every level of Iraq’s bureaucracy, Shiite clerical and tribal establishments, and security and political apparatuses. The war turned a large part of Iraq into an...