A vanished Mona Lisa. Nazi looters emptying European museums. Masked thieves posing as Boston cops. For more than a century, major art thefts have exposed startling weaknesses in the institutions meant to protect the world’s cultural treasures. Stolen paintings, sculptures and jewels have vanished from national museums, churches and private collections—sometimes for decades, sometimes forever. The motives have ranged from profit and politics to misguided patriotism. And over time, the crimes themselves have grown more sophisticated, exposing both the vulnerability and the enduring allure of objects the world deems masterpieces.