While Charles V presided the Diet of Worms, Francis I of France sent his general Robert de la Marck to invade the Burgundian Low Countries and supported Henry II of Navarre in a campaign to recover the Iberian Navarre. The Emperor responded by declaring Francis deprived of Milan and formed an anti-French alliance with Pope Leo X (8 May 1521), who was interested in annexing the strategic territories of Parma and Piacenza, both part of French-held Milan. Thus, the Four Years' War began. Meanwhile, Cortés besieged (May 26) and occupied (August 13) Tenochtictlan, completing the conquest of the Aztec Empire. The ship carrying the main treasure of emperor Cuauhtémoc was captured by the French corsair Jean Fleury, but 120,00 ducats' worth of bullion reached the Imperial treasury during the Italian conflict. Renaissance Italy was described by Mercurino di Gattinara as "the principal foundation of empire" and both Francis I and Charles V, who were considered the most powerful European monarchs of the time, aspired to primacy in the rich peninsula. As the Renaissance historian Francesco Guicciardini explained: