On July 17, 1918, in the waning months of World War I, Pvt. Adolf Hitler of the 16th Bavarian Reserve Regiment escorted two American prisoners to brigade headquarters. The encounter with his prisoners changed Hitler’s life, according to Brendan Simms in his fascinating “Hitler: A Global Biography.” From that day on, the Austrian soldier—and, soon, the war veteran-turned-racist ideologue—convinced himself that the doughboys were in fact descendants of sturdy German emigrants to the New World whom the Fatherland had allowed to slip away. They had now, in Mr. Simms’s words, “returned as avengers in the ranks of an unstoppable...