James Swan parked his old Dodge alongside the South Dakota visitor center, where grungy hippies were sprawled on a lawn and passing around a feather. The two-dozen vagabonds are planning to unleash thousands of their brethren into the Black Hills for prayer and free thinking. But Swan wasn’t feeling the peace and love. “We don’t want you here. You have no f—king respect for Lakota people!” the 54-year-old Native American yelled into a mic attached to his truck. His T-shirt bore another message: portraits of warriors who had shellacked the U.S. Army in the Battle of Little Bighorn, alongside the...