The Portuguese succession crisis refers to a period of political instability in the early 17th century, primarily between 1580 and 1640. It began when King Sebastian of Portugal died in the Battle of Alcácer Quibir without an heir, leading to a struggle for the throne. The crisis escalated when Philip II of Spain claimed the Portuguese crown, uniting the two kingdoms under Spanish rule, which many Portuguese opposed. This situation resulted in a series of conflicts and resistance movements, culminating in the Restoration War. In 1640, Portugal successfully regained its independence, marking the end of the crisis and the establishment of the Braganza dynasty as the new ruling house.