Military
Cesare's career was founded upon his father's ability to distribute patronage, along with his alliance with France (reinforced by his marriage with Charlotte d'Albret, sister of John III of Navarre), in the course of the Italian Wars. Louis XII invaded Italy in 1499; after Gian Giacomo Trivulzio had ousted its duke Ludovico Sforza, Cesare accompanied the king in his entrance into Milan.
At this point, Alexander decided to profit from the favorable situation and carve out for Cesare a state of his own in northern Italy. To this end, he declared that all his vicars in Romagna and Marche were deposed.
Cesare was appointed commander of the papal armies with a number of Italian mercenaries, supported by 300 cavalry and 4,000 Swiss infantry sent by the king of France. Alexander sent him to capture Imola and Forlì, ruled by Caterina Sforza (mother of the Medici condottiero Giovanni dalle Bande Nere). Despite being deprived of his French troops after the conquest of those two cities, Borgia returned to Rome to celebrate a triumph and to receive the title of Papal Gonfalonier from his father. In 1500, the creation of twelve new cardinals granted Alexander enough money for Cesare to hire the condottieri, Vitellozzo Vitelli, Gian Paolo Baglioni, Giulio and Paolo Orsini, and Oliverotto Euffreducci, who resumed his campaign in Romagna.
Giovanni Sforza, first husband of Cesare's sister Lucrezia, was soon ousted from Pesaro; Pandolfo Malatesta lost Rimini; Faenza surrendered, its young lord Astorre III Manfredi being later drowned in the Tiber by Cesare's order. In May 1501, the latter was created Duke of Romagna. Hired by Florence, Cesare subsequently added the lordship of Piombino to his new lands.
While his condottieri took over the siege of Piombino which ended in 1502, Cesare commanded the French troops in the sieges of Naples and Capua, defended by Prospero and Fabrizio Colonna. On 24 June 1501, Borgia's troops stormed the latter to end the siege of Capua.
In June 1502, he set out for Marche, where he was able to capture Urbino and Camerino by treason. He planned to conquer Bologna next. However, his condottieri, most notably Vitellozzo Vitelli and the Orsini brothers (Giulio, Paolo, and Francesco), feared Cesare's cruelty and set up a plot against him. Guidobaldo da Montefeltro and Giovanni Maria da Varano returned to Urbino and Camerino, and Fossombrone revolted. The fact that his subjects had enjoyed his rule thus far meant that his opponents had to work much harder than they would have liked. He eventually recalled his loyal generals to Imola, where he waited for his opponents' loose alliance to collapse. On 31 December 1502, Cesare called for a reconciliation, but imprisoned his condottieri in Senigallia, then called Sinigaglia, a feat described as a "wonderful deceiving" by historian Paolo Giovio, and had them strangled. In 1503, he conquered the Republic of San Marino.