When Philip II of Portugal (Philip III of Spain) died, he was succeeded by Philip III (and IV of Spain) who had a different approach on Portuguese issues.
Taxes raised affected mainly the Portuguese merchants (Carmo Reis 1587).
The Portuguese nobility began to lose its importance at the Spanish Cortes, and government posts in Portugal were occupied by Spaniards.
Ultimately, Philip III tried to make Portugal a royal province, and Portuguese nobles lost all of their power.

Several other problems also damaged Portuguese support of their union with Spain.
One of these was certainly the pressure from the center, especially from the Count-Duke of Olivares, towards uniformity and sharing the financial and military burden of Castile's wars in Europe.
However, the Portuguese were hardly inclined to help with that, as Spain had failed to prevent the Dutch occupation of several of Portugal's colonial holdings, despite the fact that both the Portuguese and the Spanish were nominally under the same crown.

This situation culminated in a revolution by the nobility and high bourgeoisie on December 1, 1640, 60 years after the crowning of Philip I. This revolution, while foreseeable, was most immediately sparked by a popular Catalan Revolt against the Crown.
The plot was planned by Antão Vaz de Almada, Miguel de Almeida and João Pinto Ribeiro.
They, together with several associates, known as the Forty Conspirators, took advantage of the fact that the Castilian troops were occupied in the other side of the peninsula.
The rebels killed Secretary of State Miguel de Vasconcelos and imprisoned the king's cousin, the Duchess of Mantua, who had governed Portugal in his name.
The moment was well chosen, as Philip's troops were at the time fighting the Thirty Years' War in addition to the previously mentioned revolution in Catalonia.

The support of the people became apparent almost immediately and soon John, 8th Duke of Braganza, was acclaimed King of Portugal throughout the country as John IV.
By December 2, 1640, John had already sent a letter to the Municipal Chamber of Évora as sovereign of the country.
When Philip II of Portugal (Philip III of Spain) died, he was succeeded by Philip III (and IV of Spain) who had a different approach on Portuguese issues. Taxes raised affected mainly the Portuguese merchants (Carmo Reis 1587). The Portuguese nobility began to lose its importance at the Spanish Cortes, and government posts in Portugal were occupied by Spaniards. Ultimately, Philip III tried to make Portugal a royal province, and Portuguese nobles lost all of their power.
Several other problems also damaged Portuguese support of their union with Spain. One of these was certainly the pressure from the center, especially from the Count-Duke of Olivares, towards uniformity and sharing the financial and military burden of Castile's wars in Europe. However, the Portuguese were hardly inclined to help with that, as Spain had failed to prevent the Dutch occupation of several of Portugal's colonial holdings, despite the fact that both the Portuguese and the Spanish were nominally under the same crown.
This situation culminated in a revolution by the nobility and high bourgeoisie on December 1, 1640, 60 years after the crowning of Philip I. This revolution, while foreseeable, was most immediately sparked by a popular Catalan Revolt against the Crown. The plot was planned by Antão Vaz de Almada, Miguel de Almeida and João Pinto Ribeiro. They, together with several associates, known as the Forty Conspirators, took advantage of the fact that the Castilian troops were occupied in the other side of the peninsula. The rebels killed Secretary of State Miguel de Vasconcelos and imprisoned the king's cousin, the Duchess of Mantua, who had governed Portugal in his name. The moment was well chosen, as Philip's troops were at the time fighting the Thirty Years' War in addition to the previously mentioned revolution in Catalonia.
The support of the people became apparent almost immediately and soon John, 8th Duke of Braganza, was acclaimed King of Portugal throughout the country as John IV. By December 2, 1640, John had already sent a letter to the Municipal Chamber of Évora as sovereign of the country.
Iberian Union