From Wikipedia:
The Benin Bronzes are a collection of more than 1,000 brass plaques from the royal palace of the Kingdom of Benin. They were seized by a British force in the Punitive Expedition of 1897 and given to the British Foreign Office. Around 200 of these were then passed on to the British Museum in London, while the remainder were divided between a variety of collections.

The seizure of the Bronzes led to a greater appreciation in Europe for African culture. Bronzes are now believed to have been cast in Benin since the thirteenth century, and some in the collection date from the fifteenth and sixteenth centuries.

The Bronzes depict a variety of scenes, including animals, fish, humans and scenes of court life. They were cast in matching pairs (although each was individually made). It is thought that they were originally nailed to walls and pillars in the palace as decoration, some possibly also offering instructive scenes of protocol.

Nigeria, which includes the area of the Kingdom of Benin, bought around 50 Bronzes from the British Museum between the 1950s and 1970s, and has repeatedly called for the return of the remainder, in a case which parallels that of the Elgin Marbles.