The war must have seemed very far away from Monticello on those evenings in 1779 when Thomas Jefferson joined a Hessian prisoner of war in a violin duet, with Martha Jefferson accompanying them on the pianoforte, while Baroness Riedesel, the regal wife of the captive Hessian commander, led the party in dances. Jefferson that January had welcomed the arrival in the Charlottesville, Va., area of nearly 4,000 British and Hessian prisoners taken in the Battle of Saratoga, believing they would provide a boost to the local economy.

He...