Border tensions have long persisted between Brazil and other states in South America's Southern Cone. As Brazil's population migrated west into the country's hinterland, Brazilian citizens have migrated across the western border and settled in neighboring states. Conflicts periodically have erupted between locals and migrants, between migrants and host countries' governments and between host governments and the Brazilian government. Brazilian migration to border 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, the Amazon Rainforest hinders Brazilian migration to its northern neighbors of Colombia, Venezuela and Peru. 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. 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.
