For even towards the end of the administration of D. Martinez de la Vega clouds had arisen upon the political horizon of Europe which had begun to cast their shadows over the colonies. The slave-trade sanctioned by the famous Assiento agreement gave rise to more and more serious tension between the governments of England and of Spain. In order to execute that part of the Treaty of Utrecht which related to the importation of negro slaves into Spanish America, the British government had encouraged the formation of a company, the Compania de la Mar del Sud, or South Sea Company, which was to act as agent of the assientists. It consisted of men holding the large national debt of Great Britain and had received a grant for the exclusive trade of the South Seas. But since Spain was in possession of a great proportion of the coast in that part of the world and had so far enjoyed a monopoly of its trade, the South Sea Company derived no benefit from that grant, unless the commercial activity of Spanish America could be paralyzed. The slave-trade with its clandestine opportunities for contraband, offered the South Sea Company possibilities to undermine Spanish trade. The slavers, as the slave-carrying vessels were called, being protected by passports issued by their contractors, were not slow in getting into communication with those elements in the Spanish colonies that placed their personal profit above their duty to the country under the protection of which they lived, and had no difficulty in delivering cargoes of divers merchandise while they unloaded their human freight. Moreover they never returned to Europe in ballast, but carried a correspondingly large cargo of West Indian goods of which they disposed in European ports.