In 2003, in the run-up to the U.S.-led invasion of Iraq, the Iranian regime was ridden with anxiety. President George W. Bush had included Iran in his post-9/11 “axis of evil” in a famous 2002 speech. I interviewed many Iranian officials at the time as a Tehran-based analyst with the International Crisis Group, and I vividly remember their fear that the U.S. might turn next to Tehran.

In those anxious days, Gen. Qassem Soleimani—the powerful commander of Iran’s Quds Force, who was killed this week by a U.S. airstrike in Baghdad—performed...