One of the mortars in the first round of fire early Tuesday struck the yard of a kindergarten, drawing angry responses from Israeli leaders, although no children were in the preschool at the time. Within hours, Israel’s military responded with airstrikes on sites across Gaza. It said the targets were military training bases belonging to both Hamas, the militant Islamist group that rules the Gaza Strip, and Palestinian Islamic Jihad, another militant group that Israel said was responsible for Tuesday’s salvo. The sound of sirens and reports of Iron Dome taking down mortars from the skies over southern Israel continued...