In order to do this more effectually, he cut off from Spain every possible ally by skilful diplomatic intrigues, the study of which would give a useful illustration of strategy in the realm of politics, but he made two serious mistakes to the injury of the sea power of France. Portugal had until twenty years before been united to the crown of Spain, and the claim to it had not been surrendered. Louis considered that were Spain to regain that kingdom she would be too strong for him easily to carry out his aims. Among other means of prevention he promoted a marriage between Charles II. and the Infanta of Portugal, in consequence of which Portugal ceded to England, Bombay in India, and Tangiers in the Straits of Gibraltar, which was reputed an excellent port. We see here a French king, in his eagerness for extension by land, inviting England to the Mediterranean, and forwarding her alliance with Portugal. The latter was the more curious, as Louis already foresaw the failure of the Spanish royal house, and should rather have wished the union of the peninsular kingdoms. As a matter of fact, Portugal became a dependent and outpost of England, by which she readily landed in the Peninsula down to the days of Napoleon. Indeed, if independent of Spain, she is too weak not to be under the control of the power that rules the sea and so has readiest access to her. Louis continued to support her against Spain, and secured her independence. He also interfered with the Dutch, and compelled them to restore Brazil, which they had taken from the Portuguese. On the other hand, Louis obtained from Charles II. the cession of Dunkirk on the Channel, which had been seized and used by Cromwell. This surrender was made for money, and was inexcusable from the maritime point of view. Dunkirk was for the English a bridge-head into France. To France it became a haven for privateers, the bane of England's commerce in the Channel and the North Sea. As the French sea power waned, England in treaty after treaty exacted the dismantling of the works of Dunkirk, which it may be said in passing was the home port of the celebrated Jean Bart and other great French privateersmen.