Based on information found in manuscripts available at the Medina Library (located within The National Library of Chile) and in the General Archive of the Indies (Seville), this article analyses the geostrategic infl uence of Chile over the South Sea in 1750, by means of a plan to permanently occupy Tenquehuen Island, in the Chonos Archipelago, as well as Robinson Crusoe Island, in the Juan Fernandez Archipelago those which -according to the Court of Madrid- would soon be occupied by the British crown. Centered on a plan implemented from the province of Chiloé, the issue of the failed fort in Inche is looked at as a clear example that the realities of Latin American borders differed from the plans devised in the Capitals. The article concludes that the actions carried out in Chiloé during 1750, give testimony to the level of knowledge and the usual ignorant and vacant manner regarding the isolated territory, from Madrid, Lima and Santiago.