PARIS—In March of 2012, three off-duty French soldiers were assassinated in southern France, the first in Toulouse, the others, four days later, as they withdrew money from an ATM outside their base in Montauban. All were Arabs; a black soldier was paralyzed in the second attack. A weekend passed, and on the following Monday, a man armed with two automatic handguns parked his motor scooter outside a Jewish primary school and began to fire.