Missouri had a large Union presence throughout the Civil War, but was also inhabited by many civilians whose sympathies lay with the Confederacy.
From July 1861 until the end of the war, the state suffered up to 25,000 deaths from guerrilla warfare, more than any other state.
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.
Quantrill was at the time the most prominent guerrilla leader in the Kansas–Missouri area.
In early 1863, William and Jim Anderson traveled to Jackson County, Missouri, to join him.
William Anderson was initially given a chilly reception from other raiders, who perceived him to be brash and overconfident.

In May 1863, Anderson joined members of Quantrill's Raiders on a foray near Council Grove, Kansas, in which they robbed a store 15 miles (24 km) west of the town.
After the robbery, the group was intercepted by a United States Marshal accompanied by a large posse, about 150 miles (240 km) from the Kansas–Missouri border.
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.
Castel and Goodrich speculated that this raid may have given Quantrill the idea of launching an attack deep in Kansas, as it demonstrated that the state's border was poorly defended and that guerrillas could travel deep into the state's interior before Union forces were alerted.

In early summer 1863, Anderson was made a lieutenant, serving in a unit led by George M. Todd.
In June and July, Anderson took part in several raids that killed Union soldiers, in Westport, Kansas City and Lafayette County, Missouri.
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.
He commanded 30–40 men, one of whom was Archie Clement, an 18-year-old with a predilection for torture and mutilation who was loyal only to Anderson.
By late July, Anderson led groups of guerrillas on raids and was often pursued by Union volunteer cavalry.
Anderson was under Quantrill's command, but independently organized some attacks.

Quantrill's Raiders had an extensive support network in Missouri that provided them with numerous hiding places.
Anderson's sisters aided the guerrillas by gathering information inside Union-controlled territory.
In August 1863, however, Union General Thomas Ewing, Jr. attempted to thwart the guerrillas by arresting their female relatives, and Anderson's sisters were confined in a three-story building on Grand Avenue in Kansas City with a number of other girls.
While they were confined, the building collapsed, killing one of Anderson's sisters.
In the aftermath, rumors that the building had been intentionally sabotaged by Union soldiers spread quickly; Anderson was convinced that 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.
Castel and Goodrich maintain that by then killing had become more than a means to an end for Anderson: it became an end in itself.
Missouri had a large Union presence throughout the Civil War, but was also inhabited by many civilians whose sympathies lay with the Confederacy. From July 1861 until the end of the war, the state suffered up to 25,000 deaths from guerrilla warfare, more than any other state. 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. Quantrill was at the time the most prominent guerrilla leader in the Kansas–Missouri area. In early 1863, William and Jim Anderson traveled to Jackson County, Missouri, to join him. William Anderson was initially given a chilly reception from other raiders, who perceived him to be brash and overconfident.
In May 1863, Anderson joined members of Quantrill's Raiders on a foray near Council Grove, Kansas, in which they robbed a store 15 miles (24 km) west of the town. After the robbery, the group was intercepted by a United States Marshal accompanied by a large posse, about 150 miles (240 km) from the Kansas–Missouri border. 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. Castel and Goodrich speculated that this raid may have given Quantrill the idea of launching an attack deep in Kansas, as it demonstrated that the state's border was poorly defended and that guerrillas could travel deep into the state's interior before Union forces were alerted.
In early summer 1863, Anderson was made a lieutenant, serving in a unit led by George M. Todd. In June and July, Anderson took part in several raids that killed Union soldiers, in Westport, Kansas City and Lafayette County, Missouri. 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. He commanded 30–40 men, one of whom was Archie Clement, an 18-year-old with a predilection for torture and mutilation who was loyal only to Anderson. By late July, Anderson led groups of guerrillas on raids and was often pursued by Union volunteer cavalry. Anderson was under Quantrill's command, but independently organized some attacks.
Quantrill's Raiders had an extensive support network in Missouri that provided them with numerous hiding places. Anderson's sisters aided the guerrillas by gathering information inside Union-controlled territory. In August 1863, however, Union General Thomas Ewing, Jr. attempted to thwart the guerrillas by arresting their female relatives, and Anderson's sisters were confined in a three-story building on Grand Avenue in Kansas City with a number of other girls. While they were confined, the building collapsed, killing one of Anderson's sisters. In the aftermath, rumors that the building had been intentionally sabotaged by Union soldiers spread quickly; Anderson was convinced that 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. Castel and Goodrich maintain that by then killing had become more than a means to an end for Anderson: it became an end in itself.
William T. Anderson