The surprise return of radical Shiite cleric Moqtada al-Sadr to Iraq on Wednesday is a sign of rising Iranian influence and diminishing American political clout in Iraq.

The triumphant homecoming of the fiery anti-American leader, who incited two bloody uprisings against U.S. forces after the 2003 invasion, could also foreshadow growing Iraqi political instability. The sad fact is that the rabble-rousing Islamist, who fled to Iran after his Mahdi militia was crushed, has now returned to Iraq more powerful than ever. His followers won 39 seats in last year’s parliamentary elections and were invited to join Iraq’s new government of national unity in a deal brokered by Iran last fall.

Iraqi Prime Minister Nouri al-Maliki, who has a history of bitter clashes with al-Sadr, opted to enter an uncomfortable political alliance with the Sadrist faction in order to win a second term in office. Some observers hope that al-Sadr has evolved into a more moderate political leader, noting that he had previously said he would not return until U.S. troops had left Iraq.

But the inflammatory cleric remains implacably opposed to the United States and has become a close ally of Iran, where he has spent his time in exile, reportedly studying Shiite religious doctrine under the tutelage of Iran’s radical ayatollahs. It is more likely that he has returned to consolidate control over his fragmenting political movement, prevent the rise of political rivals, and block any extension or modification of the status of forces agreement, which requires that all U.S. troops to leave Iraq by the end of the year.

The release of hundreds of al-Sadr’s followers from jail has strengthened his Mahdi militia and given him greater leverage to block any extension of the status of forces agreement, and it could revive the prospects for future sectarian bloodletting. The inclusion of Sadrists in the ruling coalition also strengthens Tehran’s influence over Baghdad. Iran’s new Foreign Minister, Ali Akbar Salehi, visited Baghdad earlier this week to cement close ties with Iraq’s new government.

Meanwhile, the Obama Administration remains more focused on a quick exit from Iraq than on preserving a pro-American partner in Baghdad. Iran is determined to fill the resulting power vacuum. And it has found a willing partner in Moqtada al-Sadr.