As Hitler launched his invasion of Poland in 1939, he instructed his commanders “to send to death mercilessly and without compassion, men, women and children of Polish derivation and language.” He assured his staff the world would raise little objection: “Who, after all, speaks today of the annihilation of the Armenians?”

That was a reference to the systematic destruction of the Armenian population by the Ottoman Turks beginning in 1915. World powers had offered little resistance to the slaughter as it occurred. Later, Turkey’s...