If the Italian Renaissance was a dream, the late Renaissance in Italy was a nightmare. As a checkerboard of independent principalities, Italy was tantalizing territory for the powerful, centralized monarchies newly arrived on the Continental scene. The French monarchy, which had consolidated its domestic power following the Hundred Years’ War, and the Catholic monarchs of Spain—Ferdinand of Aragon and Isabella of Castile, who in 1492 had completed the reconquista of the Spanish peninsula by taking the last Muslim stronghold, Granada—regarded Italy as a region of vast wealth and strategic importance. For both France and Spain, the prospect of expansion into the Italian peninsula held a powerful appeal.

Whenever they faced a crisis, all too often the Italian city-states called upon some foreign ally to tip the balance of power in their favor. In 1494, the Duke of Milan, Ludovico Sforza, “Il Moro,” invited the King of France to invade Naples in order to punish his old enemy, Ferrante II, the Aragonese king of Naples. French King Charles VIII, who had just come of age, wanted to do something bold, and Il Moro gave him his chance. Pressing an ancient claim to the Kingdom of Naples, Charles gathered an army of 25,000 men and invaded the peninsula. Unexpectedly, city after city capitulated to him, and within a few short months Charles marched into Naples, having conquered virtually all of Italy.

The French invasion heralded a period of foreign involvement in Italian aff airs that would last for more than 60 years. Machiavelli found the situation so desperate that he exhort...