Brazilian migration to other La Plata River states frequently occurs in large part because of regional geography. The terrain between Brazil, Paraguay and Uruguay is easily traversed, facilitating the natural progression of migrants over a vaguely demarcated and loosely monitored border. In contrast, Brazilian migration to its northern neighbors of Colombia, Venezuela and Peru is hindered by the Amazon Rainforest.
Differentiating population densities on either side of the border also play a role in Brazilian migration patterns. Besides the buffer of the rainforest, relatively low population densities on both sides of the border prevent heavy migration into countries to Brazil's north. Brazil's westward expansion has gradually increased the population density along its southwestern border, from Acre down to Parana, across the border from low population density areas, promoting a natural spillover into neighboring countries. In Paraguay, a population decline after the War of the Triple Alliance, also called the Paraguayan War, prompted the government to welcome Brazilian migrants in the 1960s and 1970s to boost agriculture in eastern states. In Bolivia, migration has been a natural product of proximity and opportunity as a result of Bolivia's loosely patrolled borders and low population density along the border.
Brazilian migration can create political turmoil for neighboring countries. The presence of foreign citizens in border regions can prompt outbursts of public dissatisfaction and instances of government intervention in standing issues like land reform and criminal activity. Furthermore, the migrants' presence blurs the line of sovereign control and often has served as justification for Brazilian intervention, such as the invasion of Uruguay in 1864, or the annexation of land, such as the purchase of Acre state from Bolivia. In addition, tensions tend to build between local populations and Brazilian migrants and their descendants, who often continue speaking Portuguese and remain socially separate from Spanish-speaking communities, as seen in the case of Paraguay's populations of Brazilian descent.
The Brazilian central government treats delicately the issue of Brazilian migrants and the occasional outbursts of violence. Though the Brazilian migrant populations have occasionally asked Brazilian lawmakers for assistance and protection, such intervention could hurt Brasilia's diplomatic relations with its neighbors, which could perceive the intervention as intrusive or domineering.
Indeed, the role of Brazilian migration patterns in the country's border countries should not be overstated, given that the majority of Brazilian migration is to other parts of the world. However, the continuing flow of migrants to Brazil's south and west has built a growing population of people who increasingly are linguistically and culturally tied to Brazil even if they are not Brazilian citizens. This is particularly true in regions where centralized national control is ineffective or politically distant. This can be seen in the northern region of Uruguay where the population has always felt a bit left out by Montevideo but is economically tied to Brazil and experiences continuous back-and-forth migration.
As a result of the natural cross-border flow of migrants, Brazil's borders with Spanish-speaking countries of the Southern Cone have experienced a blending of language, culture and politics. With Portuguese-speaking Brazilians increasingly influencing border zones in the states buffering Brazil and Argentina, Brazil, normally very inward-looking state, must increasingly reach out to and build close relationships with the governments of Bolivia, Uruguay and Paraguay. While this is not a phenomenon unique to Brazil and its neighbors, it is one source of Brazil's growing dominance in South America, particularly regarding its Southern Cone rival, Argentina.
