In 1864, Paraguay decided to side with the Blanco government of Uruguay, who were fighting a civil war against the Colorado faction, backed by Argentina and Brazil. To alleviate pressure on the Blancos, the Paraguayan military mobilized and attacked both Brazilian and Argentinian provinces. Unfortunately, by April 1865, the Blancos were defeated when Montevideo was surrounded and the Colorados took power.
Paraguay now found itself at war with the Triple Alliance of Argentina, Brazil, and Uruguay. The Paraguayan invasions had won many battles, but by the end of 1865 the tide was turning. In April 1866, Allied troops crossed the Parana River and bombarded Paso de la Patria with a Brazilian naval fleet. This one-sided action involved an amphibious assault. Next, the allies attacked and successfully took the outpost of Fort Itapiru. In response, the Paraguayans had retreated into the great marshland of Estero Bellaco, which made up part of
