Two days after Germany invaded Poland on Sept. 1, 1939, Britain and France declared war on the Nazi regime. On the evening of Sept. 3, a German U-boat torpedoed the British passenger liner SS Athenia, a sign that Hitler intended to wage a ruthless war at sea. The sinking triggered ample public outrage, but little immediate Allied military action took place. Over the next eight months, the only military land operation on the Western Front was France’s Saar Offensive, a short-lived incursion across Germany’s western border. This ominously quiet period came to be called the “Phoney War”—a term believed to have been coined by American senator William Borah. The invasion of Poland happened a week after Germany and the Soviet Union had signed a non-aggression pact. By the middle of Septembe...