A billboard depicting the late Iranian leader Ayatollah Ruhollah Khomeini was erected recently on the edge of Baghdad’s Firdaus Square. (Liz Sly/ The Washington Post)

Perhaps nothing illuminates more starkly the transformation underway in Iraq than the billboard depicting the late Iranian leader Ayatollah Ruhollah Khomeini erected recently on the edge of Baghdad’s Firdaus Square. The portrait obscures the view of the plinth where a giant statue of Saddam Hussein once stood, until U.S. Marines pulled it down in 2003.

The 2003 event was a profoundly symbolic moment that seemed to capture the swift triumph of American troops over Hussein’s crumbling army. It also signaled the start of Iraq’s steady drift into the orbit of Iranian influence, a trend that has accelerated dramatically since the surge into northern Iraq by the Islamic State last summer.

The billboard is one of many put up around the streets of Baghdad advertising the multiple Shiite militias that have emerged to battle the Islamic State, many of them with support from Iran. This one advertises the Resurrection of Hussein Brigade, a newly formed group that Iraqis say was directly created by Iran. It also features the portrait of Iran’s current supreme leader, Ayatollah Ali Khamenei, Khomeini’s successor.

In the background is the Palestine Hotel, where many of the journalists who covered the U.S. invasion stayed, and where U.S. Marines set up one of their first offices.

Much of the Sunni-Shiite rivalry tearing Iraq apart today is rooted in the animosity between Saddam Hussein and Khomeini, the architect of the Iranian revolution. Under Hussein’s Sunni-dominated regime, Iraq waged an eight-year war against Shiite Iran in the 1980s, in which more than a million Iranians and Iraqis died. Iraq was backed in its endeavor by the Sunni states of the Persian Gulf as well as the United States, which saw Hussein’s Iraq as a bulwark against the expansionist ambitions of Khomeini’s new Shiite republic.

Thirty-five years after Iraq launched the war, both men are dead. (Khomeini died in 1989, and Hussein was executed in 2006.) But it is the image of Khomeini that endures, in the heart of the capital of Iraq.