The United States may not exist if not for their sacrifice, but their burial place remains a mystery. Military author Patrick O’Donnell is calling on Brooklyn to find the graves of the so-called “Maryland 400,” a mostly Irish Catholic regiment in the Revolutionary War. Mr. O’Donnell’s new book, “Washington’s Immortals,” dives into the story of these heroes and their connection to historic Brooklyn locations like Green-Wood Cemetery, the Old Stone House, and what is now an American Legion post in Park Slope. Currents Correspondent Katie Breidenbach reports.