Through schools, newspapers, and military quarters from Argentina and Chile, it is taught that the neighbor country is always the expansionist and lands'robber one. To argentinians, all lands to the south of the Bío-Bío belonged to Río de la Plata's Viceroyalty, and therefore, they should be now argentinian; to chileans, the Patagonia/would have been owned by Chile until the 1881's Treaty, which Chile/would have signed under pressure, since the Pacific War was taking place at that moment. This article shows that both positions are untrue, since some documents, antecedents and royal dispositions haven't been taken into consideration by these two countries; antecedents showing new information which supports the opponent positions. Serious mistakes are described and explained through new documentary evidence, mistakes which are already standardized in both, argentinian's and chilean's historiography.Through schools, newspapers, and military quarters from Argentina and Chile, it is taught that the neighbor country is always the expansionist and lands'robber one. To argentinians, all lands to the south of the Bío-Bío belonged to Río de la Plata's Viceroyalty, and therefore, they should be now argentinian; to chileans, the Patagonia/would have been owned by Chile until the 1881's Treaty, which Chile/would have signed under pressure, since the Pacific War was taking place at that moment. This article shows that both positions are untrue, since some documents, antecedents and royal dispositions haven't been taken into consideration by these two countries; antecedents showing new information which supports the opponent positions. Serious mistakes are described and explained through new documentary evidence, mistakes which are already standardized in both, argentinian's and chilean's historiography.