The United States had never faced a clash quite like the one that began with the events at Fort Sumter in Charleston Harbor. While wars and militarism were familiar to many of the central participants in The Demon of Unrest—several were veterans of the Mexican-American War and other conflicts—the secession crisis for the first time turned Americans against Americans in a violent outburst that would eventually take hundreds of thousands of lives. People from both the North and South felt deep loyalty to many things: Their families, comrades, states, and countries. By focusing on how those different loyalties can conflict with one another, Erik Larson explores the problem of loyalty when faced with Civil War.

In previous wars, loyalties aligned. Military figures like Major Anderson, Captain Doubleday, and Robert E. Lee knew which side to fight for, since they could remain loyal to their families, states, and countries all at once. In 1860 and 1861, however, things were different. As the tension over enslavement escalated into secession and the budding formation of a new country—the Confederacy—people had to choose which of their loyalties mattered to them the most, with the understanding that they might have to fight and kill their friends and neighbors on the other side.