See here, here and here for various perspectives on the story.
The bronzes, depicting a rabbit and and a rat, formerly in the collection of Yves St Laurent, were, apparently, looted from the Summer Palace by British and French troops in 1860. It's worth bearing in mind here, that these were, by no means, the only objects removed, under rather dubious circumstances, from China by Europeans during the nineteenth and early twentieth centuries. Indeed, the famous lacquered throne that has pride of place in the Victoria and Albert Museum's T.T. Tsui Gallery, was itself taken from the Nan Haizi (Southern Ponds hunting park, south of Beijing) by Russian troops in the aftermath of the Boxer Uprising (1900-01), before being purchased, legitimately, by the museum from the dealer Spink & Son in 1922.
If you're interested in the looted imperial objects from China you can do worse than have a look at James Hevia's writings on the subject.