The Uruguayan War was a civil war in Uruguay that was fought between the Brazilian and Argentine-backed Colorados and the Paraguayan-backed Blancos. Paraguay's entry into the war in November 1864 triggered the start of the deadly Paraguayan War.
Following its independence from Brazil in 1828, Uruguay was troubled by strife between the liberal Colorados and the conservative Blancos, who engaged in internecine rebellion whenever the other party dominated the government. In 1863, Colorado leader Venancio Flores launched the "Liberating Crusade" insurrection against President Bernardo Berro, who presided over a Colorado-Blanco coalition government. Flores was aided by Argentine president Bartolome Mitre, who provided him with supplies, volunteers, and river transport in exchange for the Colorados supporting Buenos Aires in its own civil war with the Federalist faction in Argentine.
The Colorados quickly abandoned their coalition with Berro to join Flores' ranks, and the civil war quickly escalated and destabilized the entire region. The now purely Blanco government sought an alliance with Paraguayan dictator Francisco Solano Lopez and with Argentina's Federalists, while the Empire of Brazil intervened in the war to reestablish the security of its southern frontiers and protect the fifth of the Uruguayan population that considered itself Brazilian. Brazilian-Colorado troops took one town after another before besieging Montevideo, and the Blanco government capitulated on 20 February 1865. However, Paraguayan intervention in support of the Blancos triggered the long and costly Paraguayan War.