Soldiers in the Union Army volunteered for many reasons--to reunite the country, to put down the southern rebellion. For most, however, slavery was a peripheral issue. Sympathy for slaves often came only after the soldiers actually witnessed their plight.

In November 1863, thirty-eight men of the Minnesota Ninth Regiment responded to a fugitive slave's desperate plea by holding a train at gunpoint and liberating his wife, five children, and three other family members who were being shipped off to be sold. But this rescue happened in Missouri, where Union soldiers had firm orders...