The Franco-Spanish War (1635–1659) was fought between France and Spain, with the participation of a changing list of allies through the war. The first phase, beginning in May 1635 and ending with the 1648 Peace of Westphalia, is considered a related conflict of the Thirty Years' War. The second phase continued until 1659 when France and Spain agreed to peace terms in the Treaty of the Pyrenees. Major areas of conflict included northern Italy, the Spanish Netherlands, and the German Rhineland. In addition, France supported revolts against Spanish rule in Portugal (1640–1668), Catalonia (1640–1653) and Naples (1647), while from 1647 to 1653 Spain backed French rebels in the civil war known as the Fronde. Both also backed opposing sides in the 1639 to 1642 Piedmontese Civil War. France avoided direct participation in the Thirty Years' War until May 1635 when it declared war on Spain and the Holy Roman Empire, entering the conflict as an ally of the Dutch Republic and Sweden. After Westphalia in 1648, the war continued between Spain and France, with neither side able to achieve decisive victory. France made gains in Flanders and along the north-eastern end of the Pyrenees, but by 1658 both sides were financially exhausted, leading them to make peace in November 1659. French territorial gains were minor but strengthened the kingdom's borders; additionally, Louis XIV married Maria Theresa of Spain, eldest daughter of Philip IV of Spain. Spain retained a vast global empire and remained a leading power in Europe, but the treaty marked the beginning of a gradual loss of its European predominance, in favor of a rising France. Strategic overview 17th century Europe was dominated by the struggle between the Bourbon kings of France, and their Habsburg rivals in Spain and the Holy Roman Empire. Until the mid 20th century, the Thirty Years' War was primarily seen as a German religious conflict; in 1938, historian CV Wedgwood argued that it formed part of a wider, ongoing European struggle, with the Habsburg-Bourbon conflict at its centre. Modern historians sometimes refer to the Franco-Spanish War as a 'declared war', the formal part of a much wider contest with many different locations and participants. During the 1620s, France was threatened internally by a series of Huguenot rebellions, and externally by Habsburg possessions on its borders in the Spanish Netherlands, Franche-Comté, Alsace, Roussillon and Lorraine. Prior to 1635, it sought to weaken both branches