The commerce with the East Indies was of special value and importance. The discovery of the Cape route by Vasco di Gama, in 1497, led to the great stream of traffic between Europe and the East being diverted in the next century from its old channel in the Mediterranean and Levant to the Atlantic. The lucrative trade with the Indies was transferred from the Venetians and the Italian Republics to the Portuguese, who then became for a time the chief trading people of the world,[612] and strove to keep it entirely in their own hands. It was particularly with reference to this monopoly that the disputes about the freedom of the sea began. The _Mare Liberum_ of Grotius was specially directed against the prohibition by the Portuguese for any other nation to navigate round the Cape of Good Hope or to trade with the Indies. It has been well said by Calvo that the historical antecedents of the controversy about _mare clausum_ are to be found in the voyages of Columbus and Vasco di Gama.[613] Very soon, however, the claims of other Powers to maritime sovereignty--of Denmark, Venice, England--were similarly assailed, and the controversy became general. It may be noted that those who took part in it on the one side or the other, including some of the most learned men of their age, were in large measure inspired by patriotic motives. National interests as much as lofty ethics or legal principles were at its root. Even Grotius, notwithstanding his impassioned appeal to the conscience of the world for the liberty of the sea and the freedom of commerce, was not exempt from this weakness. It was his happy fortune that the cause he publicly advocated was equally in conformity with the growing spirit of liberty and the immediate interests of the United Provinces. Only four years later, when the Dutch had obtained a footing in the East Indies in spite of the Portuguese, they in turn wished to exclude the English from any share in the trade with that opulent region: they did not want any freedom of commerce that might tell against themselves. And then we find Grotius arguing, in London, against his own declarations in _Mare Liberum_, and in favour of commercial monopoly for his native land--a task, which, we are told, he performed “with uncommon ability.”