Throughout the Obama years, gathering momentum as a militia group, the Oath Keepers repeatedly inserted themselves into prominent public events. They turned up, for instance, in 2014 at a cattle ranch in Nevada after its owner, Cliven Bundy, engaged in an armed standoff with federal land management officials. That same year, members of the group went to Ferguson, Mo., in a self-appointed mission to protect local businesses from riots prompted by the death of Michael Brown, a Black man who was shot by the police.