During the American Civil War, James, along with his older brother Frank, became embroiled in the conflict, aligning themselves with pro-Confederate guerrillas known as "bushwhackers." Operating primarily in Missouri and Kansas, these groups were notorious for their brutal tactics against Union soldiers and civilian abolitionists. The brothers became followers of the infamous William Quantrill and "Bloody Bill" Anderson, gaining notoriety for their involvement in violent acts, including the Centralia Massacre in 1864, where numerous Union soldiers and civilians lost their lives in a bloody confrontation.