When the British Museum lent one of the sculptures known as the Elgin marbles to the State Hermitage Museum here last year, the move angered Greece, which wants the sculptures back, and set off a spirited debate about restitution. But it was also a diplomatic coup by one man: Mikhail Piotrovsky, director of the Hermitage and a skilled cultural ambassador.

Geoffrey Robertson and Amal Clooney helped Greece fight for the return of the Elgin Marbles. Britain refuses to listen, but has shocked the art world by secretly loaning one of the antiquities to Russia.