Iran moved quickly to replace the assassinated leader of the elite Quds Force with one of his top deputies and preserve the networks Qassem Soleimani had built before a U.S. airstrike killed him early Friday morning.

Supreme Leader Ali Khamenei appointed Esmail Ghaani to the role hours after a U.S. drone attacked Gen. Soleimani’s convoy shortly after leaving Baghdad’s international airport. The attack eliminated the main architect of Iran’s foreign forays, ranging from the propping up Lebanon’s Hezbollah movement to training...