There is not just one mask - there are four. From left to right, these are the masks at the Metropolitan Museum of Art in NYC: The British Museum in London, the Seattle Art Museum and the Linden Museum in Stuttgart, Germany.
All were looted by the British from the palace of the Oba (king) of Benin in the Punitive Benin Expedition of 1897. That attack brought down the Benin Empire, a major power in the region since the 11th century AD.