Portugal voluntarily entered a dynastic union (1580–1640) because the last two kings of the House of Aviz died without heirs, resulting in the Portuguese succession crisis of 1580. Philip II of Spain claimed the throne and was accepted as Philip I of Portugal. Portugal did not lose its formal independence, forming a union of kingdoms. But the joining of the two crowns deprived Portugal of an independent foreign policy, and led to its involvement in the Eighty Years' War between Spain and the Netherlands.
portugal voluntarily entered a dynastic union (1580–1640) because the last two kings of the house of aviz died without heirs resulting in the portuguese succession crisis of 1580 philip ii of spain claimed the throne and was accepted as philip i of portugal portugal did not lose its formal independence forming a union of kingdoms but the joining of the two crowns deprived portugal of an independent foreign policy and led to its involvement in the eighty years' war between spain and the netherlands