structural:summarization, linguistic:paraphrasing
Uruguayan prelude

Brazil had carried out three political and military interventions in the politically unstable Uruguay:

On 19 April 1863, Uruguayan general Venancio Flores, who was then an officer in the Argentine army as well as the leader of the Colorado Party of Uruguay, invaded his country, starting the Cruzada Libertadora with the open support of Argentina, which supplied the rebels with arms, ammunition and 2,000 men. Flores wanted to overthrow the Blanco Party government of president Bernardo Berro, which was allied with Paraguay.

Paraguayan president López sent a note to the Argentine government on 6 September 1863, asking for an explanation, but Buenos Aires denied any involvement in Uruguay. From that moment, mandatory military service was introduced in Paraguay; in February 1864, an additional 64,000 men were drafted into the army.

One year after the beginning of the Cruzada Libertadora, in April 1864, Brazilian minister José Antônio Saraiva arrived in Uruguayan waters with the Imperial Fleet, to demand payment for damages caused to Rio Grande do Sul farmers in border conflicts with Uruguayan farmers. Uruguayan president Atanasio Aguirre, from the Blanco Party, rejected the Brazilian demands, presented his own demands, and asked Paraguay for help. To settle the growing crisis, Solano López offered himself as a mediator of the Uruguayan crisis, as he was a political and diplomatic ally of the Uruguayan Blancos, but the offer was turned down by Brazil.

Brazilian soldiers on the northern borders of Uruguay started to provide help to Flores' troops and harassed Uruguayan officers, while the Imperial Fleet pressed hard on Montevideo. During the months of June–August 1864 a Cooperation Treaty was signed between Brazil and Argentina at Buenos Aires, for mutual assistance in the Plate Basin Crisis.

Brazilian minister Saraiva sent an ultimatum to the Uruguayan government on 4 August 1864: either comply with the Brazilian demands, or the Brazilian army would retaliate. The Paraguayan government was informed of all this and sent to Brazil a message, which stated in part:

The government of the Republic of Paraguay will consider any occupation of the Oriental territory as an attempt against the equilibrium of the states of the Platine Region which interests the Republic of Paraguay as a guarantee for its security, peace, and prosperity; and that it protests in the most solemn manner against the act, freeing itself for the future of every responsibility that may arise from the present declaration.

The Brazilian government, probably believing that the Paraguayan threat would be only diplomatic, answered on 1 September, stating that "they will never abandon the duty of protecting the lives and interests of Brazilian subjects." But in its answer, two days later, the Paraguayan government insisted that "if Brazil takes the measures protested against in the note of August 30th, 1864, Paraguay will be under the painful necessity of making its protest effective."

On 12 October, despite the Paraguayan notes and ultimatums, Brazilian troops under the command of general João Propício Mena Barreto invaded Uruguay. This was not the start of the Paraguayan war, however, for Paraguay continued to maintain diplomatic relations with Brazil for another month.

On 11 November the Brazilian ship Marquês de Olinda, on her routine voyage up the River Paraguay to the Brazilian Mato Grosso, and carrying the new governor of that province, docked at Asunción and took on coal. Completing the formalities, she continued on her journey. According to one source, López hesitated whether to break the peace for a whole day, saying "If we don't have a war now with Brazil, we shall have one at a less convenient time for ourselves". López then ordered the Paraguayan ship Tacuarí to pursue her and compel her return. On 12 November Tacuarí caught up with Marquês de Olinda in the vicinity of Concepción, fired across her bows, and ordered her to return to Asunción; when she arrived on the 13th, all on board were arrested. On the 12th Paraguay informed the Brazilian minister in Asunción that diplomatic relations had been broken off.

The conflict between Brazil and Uruguay was settled in February 1865. News of the war's end was brought by Pereira Pinto and met with joy in Rio de Janeiro. Brazilian emperor Pedro II found himself waylaid by a crowd of thousands in the streets amid acclamations. However, public opinion quickly changed for the worse when newspapers began running stories painting the convention of 20 February as harmful to Brazilian interests, for which the cabinet was blamed. The newly promoted Viscount of Tamandaré and Mena Barreto (now Baron of São Gabriel) had supported the peace accord. Tamandaré changed his mind soon afterward and played along with the allegations. A member of the opposition party, José Paranhos, Viscount of Rio Branco, was used as a scapegoat by the emperor and the government and was recalled in disgrace to the imperial capital. The accusation that the convention had failed to meet Brazilian interests proved to be unfounded. Not only had Paranhos managed to settle all Brazilian claims, but by preventing the death of thousands, he gained a willing and grateful Uruguayan ally instead of a dubious and resentful one, which provided Brazil with an important base of operations during the acute clash with Paraguay that shortly ensued.