The Federals should have seen it coming. No sooner had the 6th Massachusetts Infantry arrived in Baltimore, Maryland, on 19 April 1861, than local pro-Confederates attacked the regiment. The New Englanders were en route to Washington, DC, to help defend the capital, when rebel sympathizers began throwing stones, bricks, and bottles at them. Some brandished pistols and muskets. The soldiers fired a volley over the heads of the rioters. When that failed to intimidate them, the soldiers fired directly into crowd. The regiment escaped, but not before 4 soldiers and 12 citizens had died, with 36 soldiers and 49 citizens wounded. They were the first casualties of the American Civil War.

The mayor of Baltimore and the governor of Maryland denounced the attackers as a “mob,” but they may have just as accurately described them as urban guerrillas. In the days and weeks that followed, the countryside around Baltimore came alive with gangs of armed men who ripped up railroad tracks, cut telegraph lines, stole livestock, and took opportunities to ambush Union army patrols and pickets. By summer, rebel irregulars had sprung into action across the entire upper South, from Missouri to Virginia. They would not relent until after the surrender of the Confederate armies, four years later. By that time, rebel guerrillas had operated in every southern state.