In the annals of Civil War history, one overarching dispute remains unsettled: was the United States waging war against another nation or putting down an internal rebellion? In October 1861 three legal battles put this question to the test. As Mark Weitz reveals, these proceedings were instrumental in debating and ultimately shaping the Confederacy's very identity.

Weitz takes readers to courtrooms in Philadelphia and New York, where Confederate sailors caught raiding Union vessels were tried for the capital crime of piracy. Their defense argued that they were not pirates at all but...