When Hanoch Hai Cohen walked into the forest clearing near Kibbutz Re’im in southern Israel before dawn on Saturday, thousands of young people were already dancing under the night sky or camping out in nearby tents. The music festival, known as Nova, was so loud that Cohen, who is thirty-one and lives in Tel Aviv, saw the rockets before he heard their sound. A handful of police officers soon arrived and broke up the party, shouting “Color Red”—code in Israel for incoming rocket fire.As partygoers scrambled toward their cars or lay on the ground waiting for the barrage to pass, another kind of fire began. Cohen watched as four pickup trucks filled with armed militants and gunmen on motorcycles encircled the road leading out of the event venue, which was bottlenecked with cars attempting to...