To discover its mouth in the Caribbean, this river served much for trade between the Spanish provinces of Costa Rica, La Habana de Cuba, Cartagena de Indias in the current Colombia and other islands in the Caribbean; San Juan was already used for commercial purposes. Not only the Spaniards used the river for their purposes. Pirates, pirates and thieves invaded San Carlos and stole gold, riches and other valuable objects of Managua, Leon and Granada. Also took Indians as slaves. By his rivalry against the British, the Spaniards fortified San Carlos to so that the largest of its fortifications, the castle of the Immaculate Conception withstood many battles.
Battles against American filibusters, indigenous Miskito, and the British fleet took place in the san Juan River. After resisting attacks, decades later, the san Juan River started to lose interest to other Nations because of the construction of the Panama canal (Wikipedia encyclopedia) in 1,524 the Conqueror Hernan Cortes sent a letter to the Emperor Carlos V of Spain exposing that which possesses the passage between two oceans may be deemed owner of the world, never imagined that his words would determine the fate of Nicaragua, since from this moment dozens of expeditions to the North, South and all sides went out in search of the coveted Strait that connects Ocean to ocean. Precisely, that year, he began the colonization of Nicaragua, with the foundation of the cities of Leon in the West and Granada on the shores of the great Lake Cocibolca. 476 Years have passed since then, and oddly enough, those words of cuts yet the consequences are still being felt. As it is known, Nicaragua came to be coveted by the imperial powers of the era, not by how pretty that were neither handsome and strong indigenous who were Indians, but by its natural resources, but mainly because of the possibility of use as a transit between the Atlantic and the Pacific route taking advantage of the course of the mighty Rio San Juanbeginning exactly against the current City of San Carlos is the large lake Cocibolca desaguadero.