When he was only 15 years old, Jesse James and his brother Frank were already fighting for "Bloody Bill" Anderson and his guerilla Confederate soldiers: "Quantrill's Raiders." Under Anderson's command, Jesse fought in some of the most brutal "battles" of the Civil War, including the Lawrence Massacre and Centralia – two of the most notorious episodes in the War, notable for the execution-style killing perpetrated by Quantrill's Raiders.
After the Civil War ends, Jesse and his brother Frank return home to a Missouri that had been utterly devastated by the War. The Southern economy and infrastructure has been crippled by the union. And so, with no other options available to them, Jesse and Frank turn to the only line of work they've ever really known: stealing and killing.
Throughout his early career, Jesse frames himself as a crusader for the Southern cause – his fellow gang members are all Confederate veterans, and their targets (banks, stagecoaches, railroads) are all symbols of Northern establishment. When the Southern newspapers cover Jesse's exploits – sometimes even glorifying his acts as political rebellion against the Union – Jesse begins to buy into his own myth.
He's a fanatic, in a sense, but also, a severely damaged personality – thanks to the atrocities he suffered and committed during the War. But when Southern Reconstruction finally ends in 1876, Jesse's political cause is no longer relevant. And after the twin defeats of The Pinkerton Raid and Northfield, Jesse gives up his crusade and goes into hiding.
When he returns three years later, Jesse is every bit as zealous as he was before... but his motivation is no longer political. He's only in it for the money and ego... it clouds his judgment and, eventually, proves to be his undoing.