There has long been suspicion around the Nazi-era origins of one of Europe's most prestigious private art collections, acquired by arms dealer Emil Buhrle, who made his fortune during the war. The German-born industrialist became a naturalized Swiss citizen in 1937. By the time he died in 1956, he had amassed around 600 artworks, including masterpieces by Cezanne, Degas, Manet, Monet, Renoir, Rembrandt, Picasso and Van Gogh. Some had previously been looted from their Jewish owners, or sold cheaply and in haste as their owners fled the Nazis. The Buhrle Foundation, which owns the collection, has been working to clarify...