U.S. forces thought they killed a terrorist. They got the wrong man.

A U.S. drone strike in Syria last year killed a 56-year-old shepherd after confusing him for a terrorist leader, an internal investigation concluded, underscoring the Pentagon’s persistent struggle to avoid unintentional casualties despite the Biden administration’s pledge to curb such incidents.

The new assessment by U.S. Central Command, which oversees American military activity throughout the Middle East, affirms a Washington Post investigation published a year ago that cast doubt on officials’ initial public claim to have slain a senior al-Qaeda figure. A summary of the investigation’s findings was provided to The Post...