For a generation, Iranian Major General Qasem Soleimani bestrode the Middle East spreading terror and death. President Trump’s decision to order the general’s death via drone attack in Baghdad Thursday night is a great boon for the region. It is also belated justice for the hundreds of Americans whom Soleimani had a hand in killing.

One reason the U.S. could track and kill Soleimani near Baghdad International Airport was the impunity he had cultivated. The general often appeared in public, especially in Syria and Iraq, as...