SGT. ROBERT McCREDIE’s men hustled down 43rd Street, turned onto Third Avenue and stopped dead. Looking north, they beheld a sight no Manhattan police officer ever expected to see. A “ragged, coatless, heterogeneously weaponed army,” in the words of one reporter, filled the boulevard two blocks uptown. The smoke of burning factories framed this rumbling multitude.

Sergeant McCredie’s eyes focused on a handful of soldiers sprinting his way, with the mob on their heels. As the troops neared, he could see the tears in their uniforms, the bruises on their faces, the terror in their eyes.

The Civil War had come to New York.

It was the Union Army draft that touched off the four days of rioting on July 13, 1863. Or was it racism? Or immigration? Or politics? It is fitting that the largest riot in American history should have so many causes. Ever since it ended, Americans have struggled to explain why a draft rebellion, a race riot, a class war and an immigrant uprising all erupted in the middle of a civil war fought in the name of unity.

The draft was the immediate cause of the trouble. As the war went badly for the North in the spring of 1863, Union leaders worried that few of the initially optimistic volunteers would re-enlist when their terms were up in 1864. So Congress established America’s first conscription system (the Confederacy already had one).