Mapuches and araucanized Indians had for a long time pillaged the Argentine southern frontier in search for cattle that was later taken to Chile through the Camino de los chilenos.
The cattle was traded in Chile for weapons and alcoholic beverages.
These tribes had strong connections with Chile and therefore gave Chile certain influence over the pampas.
Argentine authorities feared an eventual war with Chile over the region where the natives would side the Chileans, and that the war would be therefore fought in the vicinities of Buenos Aires.

In the 1870s, Argentina built a more than 500-km long trench called Zanja de Alsina, which Argentina had undertaken during the Conquest of the Desert from 1876 to 1878 to defeat the araucanized Indians occupying northern Patagonia and which aspired to control the eastern third or, at a minimum, the eastern mouth of the strait.
Mapuches and araucanized Indians had for a long time pillaged the Argentine southern frontier in search for cattle that was later taken to Chile through the Camino de los chilenos. The cattle was traded in Chile for weapons and alcoholic beverages. These tribes had strong connections with Chile and therefore gave Chile certain influence over the pampas. Argentine authorities feared an eventual war with Chile over the region where the natives would side the Chileans, and that the war would be therefore fought in the vicinities of Buenos Aires.
In the 1870s, Argentina built a more than 500-km long trench called Zanja de Alsina, which Argentina had undertaken during the Conquest of the Desert from 1876 to 1878 to defeat the araucanized Indians occupying northern Patagonia and which aspired to control the eastern third or, at a minimum, the eastern mouth of the strait.
Boundary Treaty of 1881 between Chile and Argentina