Philip IV (Spanish: Felipe IV, 8 April 1605 – 17 September 1665) was King of Spain (as Philip IV in Castile and Philip III in Aragon) between 1621 and 1665, sovereign of the Spanish Netherlands, and King of Portugal (as Philip III, Portuguese: Filipe III) until 1640. On the eve of his death in 1665, the Spanish empire reached its territorial zenith, a then-unheard-of 12.2 million square kilometres (4.7 million square miles), but in other respects was in decline, a process to which Philip's inability to achieve successful domestic and military reform is felt to have contributed. This lack of success was not due to lack of effort on Philip and Olivares part but to a failure to comprehend the root causes of the problems.