Towards the middle of the fifteenth century, the first permanent mercenary armies were tried out in Italy, and especially in Naples and Milan. These increasingly sophisticated forms of organization of the armed forces were subjected to growing control by the state apparatus. The result of this particular process was a strong tension between lords and their mercenaries that was mainly evident in times of war. The essay aims to explore the features of this tension by considering both the defence of the soldiers' prerogatives and the authorities' need to streamline their armies. The «living voice» of the key figures (lords, mercenaries, officials, ambassadors) is analyzed here as it is reflected in rich diplomatic documents recovered through painstaking investigation. The background is the War of Succession (1459-1465) that broke out in the Kingdom of Naples after the death of Alfonso the Magnanimous, a conflict that provides an effective analysis model, due to its characteristics and its duration. What emerges is a vivid picture, in which direct testimony is used to reconstruct the material and psychological universe of mercenary troops in a crucial moment in their professional development.