Of the harrowing images televised nationwide from Saturday’s white nationalist demonstration in Charlottesville, one of the more chilling sights, amid hours of raging hatred and mayhem, was of camo-clad militiamen on the streets, girded for combat in tactical vests and toting military-style semiautomatic rifles. Photos and video of the heavily armed cadre — a relatively small force commanded by a 45-year-old machinist and long-ago Navy veteran from western Pennsylvania — spread rapidly on social media, raising fears the clash of hundreds of neo-Nazis and counterprotesters might end in a bloodbath. The show of strength was about “allegiance . . . to the...