The Empire of Brazil formally announced that it would begin reprisals against Uruguay after the Uruguayan government rejected a Brazilian diplomatic ultimatum. The announcement was made by Brazilian minister José Antônio Saraiva, signaling a shift from negotiation to military action. Although Brazil did not issue a formal declaration of war on that day, the decision authorized Brazilian naval and military commanders to act. Historians widely treat this moment as the effective beginning of the Uruguayan War (1864–1865).
