King Philip I of Portugal reigned from 1580 to 1598. He was the son of King Philip II of Spain and Queen Mary I of England. His ascension to the Portuguese throne followed a dynastic crisis after the death of King Henry of Portugal, leading to a union between Portugal and Spain under the Iberian Union. During his reign, Philip I faced resistance from Portuguese nobles and the general populace, who were unhappy with Spanish rule. His rule was marked by military conflicts, including the Portuguese Restoration War, which ultimately led to the end of the Iberian Union in 1640.