Washington, D.C., was the most heavily fortified city in North America at the close of the Civil War, but the nation’s capital possessed only the flimsiest of defenses at its start. Major John Gross Barnard, chief engineer of the Military District of Washington, began erecting fortifications on the Arlington hills as early as May 1861, but much remained undone when McClellan assumed command of the Department of the Potomac on August 17 and became responsible for the capital’s defense. In the meantime, a homeopathic physician from Pittsburgh visiting the city with his young nephew created an interesting record of Washington’s emerging defenses during that first frightening summer of the war.