In a museum storage depot in Amersfoort, the Netherlands, a 17th-century painting by a Dutch old master is packed away, unseen and unappreciated. Once the property of an elderly British-Jewish couple living in France, it was seized by Nazi collaborators during World War II and sold to Hermann Göring, Hitler’s second in command. Because of an administrative error in the war’s aftermath, it ended up in the Netherlands, where it was displayed in a museum for decades. The collectors heirs sought its return in 2006, and the country investigated the case and recommended restitution the following year. But the family...