A suspected Israeli drone strike has killed senior Hamas military commander Saleh Al-Arouri in southern Beirut. The rare precision attack targeted an office of the Palestinian movement in Dahiyeh, a suburb known as home to many supporters of the Iran-backed Hezbollah movement. Given the choice of target as well as the location and timing, the incident is seen as a major escalation by Israel, which is mired in bloody fighting in Gaza.

The assassination of Arouri, who for the past six years was deputy political bureau chief of Hamas, has significant dimensions in the context of the Gaza war and the regional confrontations it has sparked. Importantly, the killing also puts the spotlight on the triangle of relations between Hamas, Hezbollah, and Iran’s...