EXPLAINER: A look at the missile that killed al-Qaida leader

For a year, U.S. officials have been saying that taking out a terrorist threat in Afghanistan with no American troops on the ground would be difficult but not impossible. Last weekend, the U.S. did just that — killing al-Qaida leader Ayman al-Zawahri with a CIA drone strike.

Other high-profile airstrikes in the past had inadvertently killed innocent civilians. In this case, the U.S. carefully chose to use a type of Hellfire missile that greatly minimized the chance of other casualties. Although U.S. officials have not publicly confirmed which variant of the...