Upon his return to Rome, he resumed his studies, this time in law. In 1628 he received the title of doctor "in utroque jure", meaning he could practice both civil and canonical law. In the same year Ferdinand II, the Habsburg emperor, laid claim to a papal territory, the Valtellina, in the Italian alps. Pope Urban VIII raised an army to defend his territory. The Prince of Palestrina, who was also a member of the Colonna family, commanded a new regiment of the Papal army, and invited Giulio to become a lieutenant in the regiment. Since neither the regiment nor Giulio had any military experience, they were assigned to a town far from the front line. Giulio knew little of military discipline. He received a message from Rome informing that his mother was seriously ill. Without asking permission from his commander, he immediately rode to Rome, and stayed there until his mother had recovered. He was summoned before the Pope, Urban VIII, to explain why he had deserted his post. He threw himself at the feet of the Pope, and pleaded to be pardoned for his excess of loyalty to his family. The Pope was impressed by Giulio's spontaneity and eloquence, forgave his desertion, and invited him to become a Papal emissary.