After Uruguay's governing Blanco Party refuses Brazil's demands, José Antônio Saraiva announces that the Brazilian military will begin reprisals, beginning the Uruguayan War.

The Uruguayan War (10 August 1864 20 February 1865) was fought between Uruguay's governing Blanco Party and an alliance consisting of the Empire of Brazil and the Uruguayan Colorado Party, covertly supported by Argentina. Since its independence, Uruguay had been ravaged by intermittent struggles between the Colorado and Blanco factions, each attempting to seize and maintain power in turn. The Colorado leader Venancio Flores launched the Liberating Crusade in 1863, an insurrection aimed at toppling Bernardo Berro, who presided over a ColoradoBlanco coalition (fusionist) government. Flores was aided by Argentina, whose president Bartolom Mitre provided him with supplies, Argentine volunteers and river transport for troops.

The fusionism movement collapsed as the Colorados abandoned the coalition to join Flores' ranks. The Uruguayan Civil War quickly escalated, developing into a crisis of international scope that destabilized the entire region. Even before the Colorado rebellion, the Blancos within fusionism had sought an alliance with Paraguayan dictator Francisco Solano Lpez. Berro's now purely Blanco government also received support from Argentine federalists, who opposed Mitre and his Unitarians. The situation deteriorated as the Empire of Brazil was drawn into the conflict. Almost one fifth of the Uruguayan population were considered Brazilian. Some joined Flores' rebellion, spurred by discontent with Blanco government policies that they regarded as harmful to their interests. Brazil eventually decided to intervene in the Uruguayan affair to reestablish the security of its southern frontiers and its regional ascendancy.

In April 1864, Brazil sent Minister Plenipotentiary Jos Antnio Saraiva to negotiate with Atanasio Aguirre, who had succeeded Berro in Uruguay. Saraiva made an initial attempt to settle the dispute between Blancos and Colorados. Faced with Aguirre's intransigence regarding Flores' demands, the Brazilian diplomat abandoned the effort and sided with the Colorados. On 10 August 1864, after a Brazilian ultimatum was refused, Saraiva declared that Brazil's military would begin exacting reprisals. Brazil declined to acknowledge a formal state of war, and for most of its duration, the UruguayanBrazilian armed conflict was an undeclared war.

In a combined offensive against Blanco strongholds, the BrazilianColorado troops advanced through Uruguayan territory, taking one town after another. Eventually the Blancos were left isolated in Montevideo, the national capital. Faced with certain defeat, the Blanco government capitulated on 20 February 1865. The short-lived war would have been regarded as an outstanding success for Brazilian and Argentine interests, had Paraguayan intervention in support of the Blancos (with attacks upon Brazilian and Argentine provinces) not led to the long and costly Paraguayan War.

The National Party (Spanish: Partido Nacional, PN), also known as the White Party (Spanish: Partido Blanco), is a major political party in Uruguay. It was founded in 1836 by Manuel Oribe, making it the country's oldest active political party, and together with the Colorado Party, its origin dates back to the time of the creation of the Uruguayan State.

Positioned on the centre-right of the political spectrum, the National Party is ideologically liberal, nationalist, Pan-Americanist and humanist. Considering the interim co-government of the Gobierno del Cerrito headed by Manuel Oribe, and the Defense Government from Montevideo led by the Colorado Joaquín Suarez, in the middle of the Uruguayan Civil War, and with the exception of the current administration of Luis Lacalle Pou, the PN has ruled the country for 35 years interruptedly throughout its history; This includes constitutional, interim, de facto presidents, and collegiate governments. Although General Manuel Oribe is recognized as its founder, Aparicio Saravia is considered its idealist.The National Party is a defender of decentralization, and its demographic base skews toward people living in rural areas.