Just over 100 days ago, Benny Gantz was the leader of a small Israeli opposition party. Now, in a shared office inside a nondescript building within the Defense Ministry compound in Tel Aviv, “Mr.” Gantz is helping lead Israel’s war against Hamas in Gaza as a member of the war cabinet formed by Prime Minister Benjamin Netanyahu. Gantz spends his days poring over operational plans, not just of the ongoing campaign in Gaza but also of contingencies for a war that may erupt with Hezbollah, the Iranian-backed Lebanese Shiite organization, on Israel’s northern border.