Nobile Piedmontese attended the Academy, from which he graduated as a second lieutenant of cavalry in 1931. The start of the war in Ethiopia prevented him from reaching Berlin as an equestrian Olympian. He fought leading a Spahis unit and was seriously wounded in his left hand during the Battle of Selaclaclà in 1935. In 1937 in Tripoli, he organized the ceremony where Mussolini proclaimed himself the "defender of Islam" before leaving for the Spanish Civil War in command of a Moroccan cavalry tabor. Upon returning to Eritrea, he led a Group of Amhara Bands in counter-guerrilla actions, accompanied by his Eritrean concubine Kadija, the daughter of an important tribal chief. During World War II in 1941, he faced the British Gazelle Force, whom he charged from behind. The battle resulted in the loss of 800 men and marked the last cavalry charge in Africa. Guillet continued to fight under the pseudonym "Cummundar as Shaitan" (Commander Devil) even after the fall of Asmara, leading a hundred of his loyal indigenous followers. His eight-month guerrilla warfare was costly for the British, who put a bounty of 1000 gold pounds on his head. Hunted down by the end of October 1941, he assumed a false identity in Massawa, where he obtained a pass to Yemen from the British Governor. Upon reaching Hodeida, he was arrested on suspicion of being a British spy. Once recognized by the Yemenite Imam, he was appointed "Grand Marshal of the Court" and tutor to the royal guard and the Imam's children. In June 1943, he returned to Massawa and managed to embark on a ship of the Italian Red Cross. He arrived in Rome on September 3, 1943. After crossing the Gustav Line miraculously following the armistice, he reached Brindisi and placed himself at the disposal of the King. He worked in the Information Service of the Southern Kingdom and was tasked with recovering the imperial crown of Negus of Ethiopia, stolen by partisans. It was returned to Negus as a sign of reconciliation. After the war and the victory of the Republic in the 1946 Referendum, Guillet, faithful to his oath to the Crown, resigned from the army.