Since the 15th century, with the unification under the Catholic Monarchs, the system of duchies consolidated as a crucial tool in territorial politics. The creation of new noble titles was closely linked to the expansion of the State and the need to reinforce control over newly conquered territories. Upon receiving their titles, dukes often became the main administrators of entire provinces, acting as representatives of the crown in the management of those territories.
Dukes had specific responsibilities that encompassed not only local governance but also tax collection and the administration of justice. For example, in the 16th and 17th centuries, many of them were responsible for managing their own fiefs, granting them considerable autonomy in decision-making. This power was reflected in their ability to create local laws and regulations, as well as their influence in appointing local authorities, such as mayors and councilors.
One of the most emblematic cases is that of the Dukes of Alba, whose influence on territorial administration was monumental. Since the 15th century, this aristocratic family played a fundamental role in politics and warfare, especially during the reigns of Charles I and Philip II. The Duke of Alba, Fernando Álvarez de Toledo, is known not only for his ferocity in battle but also for his efficient and often feared administration in the Netherlands. His management of these territories not only reaffirmed the crown's authority but also showcased the power and responsibilities that came with the ducal title.