Prince of Foxes

In August 1500, Andrea Orsini (Tyrone Power), an artistic minor nobleman who is equally skilled with the brush, the sword, words, and women, serves the Machiavellian Prince Cesare Borgia (Orson Welles) as a soldier. Pleased with Andrea's ability to "follow my mind and keep his eyes fixed on the ultimate goal," Borgia selects Andrea to accomplish an intrigue: arrange the marriage of his widowed sister, Lucrezia (whose husband he has just had assassinated for the purpose), to Alfonso d'Este, the son of the Duke Ercole d'Este of Ferrara. By doing so, Borgia will remove Ferrara as an impediment to conquest of central Italy. In being selected, however, Andrea earns the enmity of Don Esteban Ramirez (Leslie Bradley), an ambitious mercenary captain and rival.
Andrea travels to Venice to sell some of his paintings to raise money for expenses. He meets the lovely Camilla di la Baglione (Wanda Hendrix), young wife of the elderly Count Marc Antonio Verano of Citta del Monte, and smitten with her, gallantly gives her a painting he was haggling to sell for a hundred ducats. Soon after, an assassin attempts to kill Andrea, but he thwarts the attack and spares the assassin to learn who hired him: the Duke Ercole d'Este. He hires the assassin, Mario Belli (Everett Sloane), as part of his own entourage. Resuming the mission, Andrea stops to visit the farm of a blacksmith's widow, reputed to be hiding gold stolen by her bandit son. She is actually his mother (Katina Paxinou), and he is in fact Andrea Zoppo, not the noble Orsini he pretends to be. The reunion is a rocky one, because the mother does not approve of her son's evil ways. Belli spies on them through a window. Andrea continues to Ferrara, where he succeeds in arranging the marriage by intimidating the duke and flattering Alfonso.
Andrea's next mission, again chosen over Don Esteban, is as ambassador to Citta del Monte, with orders to help Borgia conquer the mountaintop city by spring, using a romantic conquest of Camilla to facilitate a "correct" elimination of the elderly count (Felix Aylmer). Borgia secretly hires Belli to spy on Andrea and report if his loyalty wavers. Andrea learns that when the old count has a problem to solve, he climbs to a high precipice looking out over the landscape. Belli is delighted because it would make it easy to kill the count, but Andrea is moved by the older man's wisdom and love for his people. Camilla, despite her suspicions of Andrea, grows to admire his artistic soul as he paints her portrait. When Borgia, through Don Esteban, orders the count to allow passage of his army and to supply troops for it, the count defies him with the support of his people. Andrea changes sides to join his cause but Belli, a self-professed "born traitor", gives notice to Andrea and returns to Borgia. The count reveals to Andrea that he married Camilla only to protect her after her father died, and has treated her as his own daughter. He is mortally wounded in an ambush and rout of the vanguard of Borgia's army and Andrea takes up the defense.
After three months of repelling assaults, the city is at its last extremity. Don Esteban offers Camilla terms that are generous to her and her people, but would require her to surrender Andrea to Borgia. In love with Andrea, she rejects the terms, but Andrea gives himself up to Don Esteban at the price of putting the terms in writing. At a triumphal dinner with Camilla, who has begged him to spare Andrea's life, Borgia hauls in Andrea, tortured by Don Esteban, and exposes "the noble Orsini" as a peasant. He brings Andrea's mother to confirm it. Borgia orders his death by starvation but Belli, now a lieutenant in Borgia's service, protests and demands to gouge out Andrea's eyes in front of everyone and make him a blind beggar. However Belli has secretly remained loyal to Andrea and fakes the disfigurement, and Mother Zoppo takes her son home. After Borgia moves on to another campaign, Andrea and Belli plot to free the imprisoned Camilla and help the people retake their city. Belli aids Andrea in entering the castle to rescue Camilla, but the signal for the citizen uprising is given before they can make their escape. Andrea kills Don Esteban in single combat, but is about to be killed by another officer after Andrea stumbles over Esteban's corpse. However Balgioni, an officer, weary of killing for Borgia and an admirer of Andrea's, recognizes and spares him. The uprising sparks widespread resistance to Borgia, and after his fall, Andrea and Camilla are married.

Question: Who does Borgia secretly hire?
Answer: Belli