Anderson led a band of volunteer partisan raiders who targeted Union loyalists and federal soldiers in the states of Missouri and Kansas.

After a building collapse in the makeshift jail in Kansas City, Missouri killed one sister, and left another permanently maimed, Anderson devoted himself to revenge.

Anderson subsequently returned to Missouri as the leader of his own group of raiders and became the most feared guerrilla in the state, robbing and killing a large number of Union soldiers and civilian sympathizers.

Historians have made disparate appraisals of Anderson; some see him as a sadistic, psychopathic killer, while others put his actions into the perspective of the general desperation and lawlessness of the time and the brutalization effect of war.

[4] In 1857, they relocated to the Kansas Territory, traveling southwest on the Santa Fe Trail and settling 13 miles (21 km) east of Council Grove.

[6] Kansas was at the time embroiled in an ideological conflict regarding its admission to the Union as slave or free, and both pro-slavery activists and abolitionists had moved there in attempts to influence its ultimate status.

[18] On July 2, 1862, William and Jim Anderson returned to Council Grove and sent an accomplice to Baker's house claiming to be a traveler seeking supplies.

[21] Anderson and his gang subsequently traveled east of Jackson County, Missouri, avoiding territory where Quantrill operated and continuing to support themselves by robbery.

[24] Confederate General Sterling Price failed to gain control of Missouri in his 1861 offensive and retreated into Arkansas, leaving only partisan rangers and local guerrillas known as "bushwhackers" to challenge Union dominance.

[27] In May 1863, Anderson joined members of Quantrill's Raiders on a foray near Council Grove, Kansas,[27] in which they robbed a store 15 miles (24 km) west of the town.

[29] In the resulting skirmish, several raiders were captured or killed and the rest of the guerrillas, including Anderson, split into small groups to return to Missouri.

In June and July, Anderson took part in several raids that killed Union soldiers, in Westport, Kansas City and Lafayette County, Missouri.

[30] The first reference to Anderson in Official Records of the American Civil War concerns his activities at this time, describing him as the captain of a band of guerrillas.

[33] In August 1863, however, Union General Thomas Ewing, Jr. attempted to thwart the guerrillas by arresting their female relatives,[34] and Anderson's sisters were confined in a three-story building on Grand Avenue in Kansas City with a number of other girls.

[35] In the aftermath, rumors that the building had been intentionally sabotaged by Union soldiers spread quickly;[36] Anderson was convinced it had been a deliberate act.

Biographer Larry Wood wrote that Anderson's motivation shifted after the death of his sister, arguing that killing then became his focus, and an enjoyable act.

[41] Arriving in Lawrence on August 21, the guerrillas immediately killed a number of Union Army recruits and one of Anderson's men took their flag.

[53] On October 12, Quantrill and his men met General Samuel Cooper at the Canadian River and proceeded to Mineral Springs, Texas, to rest for the winter.

[56] In March 1864, at the behest of General Sterling Price, Quantrill reassembled his men, sending most of them into active duty with the regular Confederate Army.

[64] The next day, in southeast Jackson County, Anderson's group ambushed a wagon train carrying members of the Union 1st Northeast Missouri Cavalry, killing nine.

Around that time, he received further media coverage: the St. Joseph Morning Herald deemed him a "heartless scoundrel", publishing an account of his torture of a captured Union soldier.

On August 10, while traveling through Clay County, Anderson and his men engaged 25 militia members, killing five of them and forcing the rest to flee.

[87] Although they forced the Union soldiers to flee, Anderson and Jesse James were injured in the encounter and the guerrillas retired to Boone County to rest.

[117] However, Frank James, who participated in the attack, later defended the guerrillas' actions, arguing that the federal troops were marching under a black flag, indicating that they intended to show no mercy.

[123] They burned Rocheport to the ground on October 2; the town was under close scrutiny by Union forces, owing to the number of Confederate sympathizers there, but General Fisk maintained that the fire was accidental.

[139] Union military leaders assigned Lieutenant Colonel Samuel P. Cox to kill Anderson, providing him with a group of experienced soldiers.

[149] They also found his flag, and one soldier described it as "... a small affair, made of fine silk, and the corner piece containing seven silver stars.

[155] Most Confederate guerrillas had lost heart by then, owing to a cold winter and the simultaneous failure of General Price's 1864 invasion of Missouri, which ensured the state would remain securely under Union control for the rest of the war.

In 1976, the book was adapted into a film, The Outlaw Josey Wales, which portrays a man who joins Anderson's gang after his wife is killed by Union-backed raiders.

[168] In a study of 19th-century warfare, historian James Reid posited that Anderson suffered from delusional paranoia, which exacerbated his aggressive, sadistic personality.