Just as former president Donald Trump told Fox News last week that he wanted to use the U.S. military to “handle” what he called the “enemy from within” on Election Day, an obscure military policy was beginning to make the rounds on social media platforms favored by the far right. The focus? Department of Defense Directive 5240.01. The 22-page document governs military intelligence activities and is among more than a thousand different policies that outline Defense Department procedures. The Pentagon updated it at the end of September. Although military policies are routinely updated and reissued, the timing of this one—just...