In one corner, a godless young warrior, Voltaire's heralded 'philosopher-king', the It Boy of the Enlightenment. In the other, a devout if bad-tempered old composer of 'outdated' music, a scorned genius in his last years. The sparks from their brief conflict illuminate a turbulent age.

Behind the pomp and flash, Prussia's Frederick the Great was a tormented man, son of an abusive king who forced him to watch as his best friend (probably his lover) was beheaded. In what may have been one of history's crueler practical jokes, Frederick challenged 'old Bach' to a musical duel, asking...