This 1862 photograph shows Allan Pinkerton in his prime: A tough Scottish immigrant, founder of the nation's leading private detective agency, personal intellegence chief to General George B. McClellan. Though his wartime work added little to his reputation, he successfully battled the pickpockets, safecrackers, and confidence men who plagued bankers and businessmen. He compiled photographs of suspects (an innovation for his time), infiltrated criminal organizations, and operated on a strict fee-for-service basis that spared his clients from the corruption that plagued many municipal detective forces. In 1869, he suffered his first stroke, limiting his personal participation in operations, but with the aid of his sons Robert and William he continued his hands-on approach to private law enforcement.

Conventional wisdom has it that Pinkerton was hired by the railroad corporations to chase the James and Younger brothers. In reality, the railroads lost virtually nothing in train robberies, and took little interest in hunting the bandits. Express companies, which contracted with railroads to ship cash and valuables in safes in baggage cars, were the real victims. In 1874, President William B. Dinsmore of the Adams Express Company hired Pinkerton after an Adams safe on an Iron Mountain train was robbed at Gads Hill, Missouri. Soon after Pinkerton began his investigation, the outlaws killed two of his agents, along with a local man who was helping the detectives. The Adams Express subsequently withdrew, but Pinkerton continued the search on his own, concocting a plan to raid Jesse's mother's farmhouse using an incendiary device. The raid went awry when the instrument exploded, killing Jesse's half-brother and maiming his mother. In the days that followed, the bandits won widespread sympathy (including in the state legislature), and the James brothers assassinated a local farmer who had aided the operation. After these bloody events, Jesse repeatedly stated his desire to murder Pinkerton. The famed detective, on the other hand, reluctantly abandoned his own quest for revenge. It was the gravest defeat of his career.