An arresting image from the last century should haunt us in this one. In November 1940, Soviet Foreign Minister Vyacheslav Molotov arrived in Berlin to deepen Moscow’s pact with Nazi Germany, supposedly communism’s mortal enemy. “The hammer and sickle on Soviet flags fluttering alongside swastika banners provided an extraordinary spectacle in the Reich’s capital,” historian Ian Kershaw later wrote. And although the Nazi-Soviet pact collapsed months later when Hitler invaded the Soviet Union, its lesson endures: Extremists who appear to hate one another can readily make common cause against an enemy they hate even more. Today, that enemy is America....