On Sunday evening, Lt. Col. Dolev Keidar, the commander of the infantry-cadet battalion at Israel’s officers’ school, met a friend from the army. They were at the memorial near Gaza to the fallen during Operation Black Arrow, a 1955 reprisal raid in the Strip. The cadets had been assigned to security duties in the Sderot area, and the friend asked Keidar about his plans for the night.