Now in a position to potentially determine much of the country's future, the Shia leader, who has railed against both U.S. and Iranian influence, could dramatically change the landscape for major powers that have invested heavily in Iraq.
A firebrand religious leader with millions of loyal followers, Sadr gained infamy shortly after the 2003 U.S.-led invasion by directing deadly attacks against American troops with his Mahdi Army, which also attacked Iraq's Sunnis.
Lately, he has shifted his focus to anti-corruption campaigns and advocating for Iraq's poor. But he's in the rare position of opposing both the U.S. and Iran — several of his election rallies triggered chants of "Iran out!" among his followers, voicing the desire for an Iraqi state run by Iraqis.