On Dec. 13, 1864, the small, landlocked but strategically positioned country of Paraguay declared war on the Brazilian Empire. The ensuing conflict became the worst in South American history. The Paraguayan War laid the foundation of contemporary South America's geopolitical divisions. The conflict emerged as a result of Paraguay's centrality in the Platine River Basin and its position in the heartland of South America. Underlying geopolitical imperatives — most notably the need to secure access to the key waterways providing trade routes and outlets to the Atlantic — forced the national interests of Argentina, Brazil, Paraguay and Uruguay to collide in the worst armed conflict in South America's history, pitting Paraguay against what came to be known as the Triple Alliance. Today, 150 years later, the impact of the war still resonates, dominating and defining the nature of regional politics.