The Civil War is remembered for its sweeping battles: Gettysburg, Atlanta, Antietam. Less known are the small troops of men, enlisted by both sides, to fight far from the battlefields.

These ruthless soldiers relied on stealth to sneak behind enemy lines — often wearing their opponent’s uniform — and destroyed supply lines, assassinated military officials and gathered critical information.

Today, we know this kind of warfare as shadow ops — which is a specialty of military historian Patrick K. O’Donnell. A roadside marker he happened to see in rural Virginia ignited years of research into the Civil War era special forces who were tasked by President Lincoln to undertake spy operations and secrete missions against Confederate units.

This week, he joined MPR News host Kerri Miller on Big Books and Bold ideas to talk about his new book, “The Unvanquished,” which masterfully tells the story of this forgotten chapter of history.