Statute of limitations imposes a term limit to bring a claim for a specific cause of action, here replevin or conversion (money for the stolen object or recovery of the object itself). Laches is a defense for defendants who may be prejudiced by plaintiff's efforts to recover, in this instance stolen or lost art. Over the last decade, museums and collectors have been turning to technical defenses to avoid reviewing facts surrounding ownership of art displaced during World War II.
A PDF of Dowd's "Nazi Looted Art and Cocaine: When Museum Directors Take it, Call the Cops is available from Rutgers J. Law & Religion.