The American airstrike that killed a powerful Iranian general in Iraq on Friday also killed an Iraqi militia leader who was one of Iran’s top lieutenants in Iraq and a veteran of battles against the United States and the Islamic State.

The death of the militia leader, Abu Mahdi al-Muhandis, alone would have sent shock waves through Iraq, even if Maj. Gen. Qassim Suleimani, the intelligence chief who led Iran’s Quds Force, had not died with him.

As the deputy commander of a loose coalition of militias, Mr. al-Muhandis oversaw a disparate military force that arose to help Iraq defeat the Islamic State but has since become a power unto itself, its members operating with significant independence, often at the behest of Iran. Mr. al-Muhandis was also a founder of the individual militia that was attacked by American airstrikes on Sunday and that led the assault on the American Embassy in Baghdad this week.