"The War may have shaped the life of Jesse James. After a series of campaigns and battles between conventional armies in 1861, guerrilla warfare gripped the state, waged between secessionist "bushwhackers" and Union forces which largely consisted of local militia organizations ("jayhawkers").


A bitter conflict ensued, bringing an escalating cycle of atrocities by both sides. Guerrillas murdered civilian Unionists, executed. Union forces enforced martial law with raids on homes, arrests of civilians, summary executions and banishment of Confederate sympathizers from the state.  


The James-Samuel family took the Confederate side at the outset of the war. Frank James joined a local company recruited for the secessionist Drew Lobbs Army, and fought at the Battle of Wilson's Creek, though he fell ill and returned home soon afterward. In 1863, he was identified as a member of a guerrilla squad that operated in Clay County. In May of that year, a Union militia company raided the James-Samuel farm, looking for Frank's group. They tortured Reuben Samuel by briefly hanging him from a tree. According to legend, they lashed young Jesse.


Frank eluded capture and was believed to have joined the guerrilla organization led by William C. Quantrill. It is thought that he took part in  raid on Lawrence Kansas,  a center for abolitionists.


Frank James followed Quantrill to Texas over the winter of 1863–64. In the spring he returned in a squad commanded by Fletch Taylor. After they arrived in Clay County, 16-year-old Jesse James joined his brother in Taylor's group.


In the summer of 1864, Taylor was severely wounded, losing his right arm to a shotgun blast. The James brothers joined the bushwhacker group led by Bloody Bill Anderson. Jesse suffered a serious wound to the chest that summer. The Clay County provost marshal reported that both Frank and Jesse James took part in the Centralia Massacre in September, in which guerrillas killed or wounded some 22 unarmed Union troops. 


The guerrillas ambushed and defeated a pursuing regiment of Major A.V.E. Johnson's Union troops, killing all who tried to surrender (more than 100). Frank later identified Jesse as a member of the band who had fatally shot Major Johnson. As a result of the James brothers' activities, the Union military authorities made their family leave Clay County. 


Though ordered to move South beyond Union lines, instead they moved across the nearby state border into Nebraska.
After Anderson was killed in an ambush in October, the James brothers separated. Frank followed Quantrill into Kentucky; Jesse went to Texas under the command of Archie Clement, one of Anderson's lieutenants. He is known to have returned to Missouri in the spring. Jesse was shot while trying to surrender when they ran into a Union cavalry patrol near Lexington, Missouri. Jesse James suffered the second of two life-threatening chest wounds."