Today, Peter Paul Rubens is best remembered as the Old Master with a penchant for fleshy, pink nudes and baroque grandiosity. These perceptions suggest a man of unchecked libertinism, but Rubens was in fact a man of controlled appetites, with a modest disposition and a reputation for tact and discretion. Almost inevitably, given his proximity to monarchs and statesmen across Europe, he was conscripted into political service as a covert diplomat and spy; his artistic work could always provide cover for his clandestine activities.

Because...