Pierre-Augustin Caron de Beaumarchais was an eighteenth-century French inventor, famed playwright, and upstart near-aristocrat in the court of King Louis XVI. In 1776, he conceived an audacious plan to send aid to the American rebels. What's more, he convinced the king to bankroll the project, and singlehandedly carried it out. By war's end, he had supplied Washington's army with most of its weapons and powder, though he was never paid or acknowledged by the United States.

To some, he was a dashing hero--a towering intellect who saved the American Revolution. To others, he was pure...