At the start of the Civil War, Dr. William McPheeters was a distinguished physician in St. Louis, conducting unprecedented public-health research, forging new medical standards, and organizing the state's first professional associations. But Missouri was a volatile border state. Under martial law, Union authorities kept close watch on known Confederate sympathizers. McPheeters was followed, arrested, threatened, and finally, in 1862, given an ultimatum: sign an oath of allegiance to the Union or go to federal prison. McPheeters acted from principle instead, fleeing by night to Confederate...