Liechtenstein, the tiny, 253-year-old principality nestled between Austria and Switzerland, ranks near the bottom of the world's temporal powers. Its boundaries barely encompass 61 sq. mi. But in the art world, little Liechtenstein shines as one of the brightest stars in the firmament. Reason: within its confines is the richest old-master collection still in private hands, including a score of Rubenses and Van Dykes along with Raphaels, Brueghels, Titians and Tiepolos.
The most famous of Prince Franz Josef II's 1,500 oils is Leonardo da Vinci's Ginevra del Bend, a painting that is...