On March 15, 1781, American forces inflicted heavy losses on the British Army at Guilford Courthouse, North Carolina. The redcoats had seemed invincible only a few months before. Winter clouds scudded over New Windsor, New York, some 50 miles up the Hudson River from Manhattan, where Gen. George Washington was headquartered. With trees barren and snow on the ground that January 1781, it was a "dreary station," as Washington put it. The commander in chief's mood was as bleak as the landscape. Six long years into the War of Independence, his army, he admitted to Lt. Col. John Laurens,...