In 1863, the Empire of Brazil—which did not have friendly relations with Paraguay—began providing military and political support to an incipient rebellion in Uruguay led by Venancio Flores and his Colorado Party against the Blanco Party government of Bernardo Berro and his successor, Atanasio Aguirre.[14] The besieged Uruguayans repeatedly asked for military assistance from their Paraguayan allies against the Brazilian-backed rebels. López manifested his support for Aguirre's government via a letter to Brazil, in which he said that any occupation of Uruguayan lands by Brazil would be considered as an attack on Paraguay.[14]
When Brazil did not heed the letter and invaded Uruguay on 12 October 1864, López seized the Brazilian merchant steamer Marqués de Olinda in the harbor of Asunción,[15] and imprisoned the Brazilian governor of the province of Mato Grosso, who was on board. In the following month (December 1864) Lopez formally declared war on Brazil and dispatched a force to invade Mato Grosso. The force seized and sacked the town of Corumbá and took possession of the province and its diamond mines, together with an immense quantity of arms and ammunition, including enough gunpowder to last the whole Paraguayan Army for at least a year of active war.[16] However, Paraguayan forces could not or would not seize the capital city of Cuiabá, in northern Mato Grosso.
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