From https://metaculus.com/questions/13924/200-bronzes-of-britmuseum-to-nigeria-by-2030/

Benin, in present-day Nigeria, was a powerful and wealthy state that had trade relations with the Portuguese since the 15th century. Benin traded ivory, gum, pepper, and cotton cloth for the Portuguese copper, brass, and other metals. In the 17th century, Benin's Oba (king) began raiding and trading for slaves from neighboring kingdoms. Benin became one of the major hubs of the slave trade in West Africa, along with Dahomey and Oyo.

After the slave trade was abolished by Britain in 1807, Benin faced several challenges and changes. The British navy tried to suppress the illegal slave trade by patrolling the coast and intercepting slave ships. This disrupted Benin's economy and trade relations with other African states and Europe. Benin also had to deal with internal conflicts and rebellions from some of its vassal states that wanted more autonomy or independence. Benin tried to resist British colonialism by maintaining its sovereignty and culture, but it was eventually defeated by a British punitive expedition in 1897 that sacked and burned the city, looted its treasures and artworks, and deposed its Oba. Benin became part of the British protectorate of Nigeria until it gained its independence in 1960.

The Benin Bronzes were a collection of brass plaques, sculptures, and other objects looted from Benin. The bronzes are notable for their intricate and detailed designs, which often depict historical events, royal figures, and other aspects of Benin culture and history. The collection is considered to be one of the greatest examples of African art and craftsmanship, and it is highly valued by museums and collectors around the world.

According to Richard Assheton,

Colonial powers initially refused to believe an African civilisation had produced these stunning works. Today, more than 900 lie in the British Museum, and others in museums and private collections all over the world.

...

The British Museum ... has refused [to transfer away ownership of any of the bronzes], saying it is bound by law not to give away its collections.