The greatest paintings in today's most famous museums were once part of a fluid exchange determined by volatile political fortunes. In the first half of the 17th century, masterpieces by Titian, Raphael, and Leonardo, among others, were the objects of fervent pursuit by art connoisseurs. Francis Haskell traces the fate of collections extracted from Italy, Spain, and France by King Charles I and his circle, which, after a brief stay in Britain,...