PHILADELPHIA — They arrived in yellow rental trucks, unfurled their flags, and readied shields and smoke bombs. The hour was late, and the symbolism was unsettling: As the clock inched close to midnight on July 3, about 200 members of the white nationalist group Patriot Front marched through downtown Philadelphia, past Independence Hall and other historic landmarks, while chanting, “Take America back!” If the demonstration was meant to be a show of strength for the organization, it ended meekly. After scuffling with a handful of counterprotesters, the Patriot Front members retreated into their...