Capture of the steamer Marquês de Olinda

The capture of the steamer Marquês de Olinda was a Paraguayan naval action carried out on 12 November 1864, in Potrero-Poña, on the Paraguay River, which consisted of the capture of the Brazilian merchant ship and all on board. Paraguayan president Solano López ordered the imprisonment in response to the Brazilian invasion of Uruguay, before any formal declaration of war between Paraguay and the Empire of Brazil.

Since the time when Carlos Antônio López ruled Paraguay, there was already disagreement between both countries, albeit peacefully, on territorial and border issues. With the rise of Solano López to power, these disagreements became more aggressive, as López expanded his foreign policy, reaching an understanding with Argentine and Uruguayan leaders in order to facilitate Paraguay's access to the sea through the Río de la Plata. Despite this, there was nothing between the Empire of Brazil and Paraguay to justify any military action on the part of those involved. However, with the Uruguayan War underway, the Uruguayans tried to pressure López to help them against the Brazilians by sending agents to persuade him and also to gather information that was relevant to the Uruguayan cause.

One of the agents discovered that Marquês de Olinda, en route to Corumbá, was carrying the newly appointed president of the Mato Grosso province, and informed López the ship carried "armaments and valuable cargo", advising him to seize it. It was a Uruguayan attempt to get the Paraguayans to act in order to distract Brazil from its territory. López followed the advice and imprisoned the ship on 12 November 1864, before it had left Paraguay. All Brazilian passengers, crew and officers, were incarcerated, and most of the latter died from torture, starvation and lack of medical care. Only two officers survived arrest, being released in 1869.

The capture of the Marquês de Olinda triggered events that led to the beginning of the Paraguayan War, although the true origin of the conflict is a reason for disagreement among historians. After the capture, the Paraguayans converted the vessel into a warship and used it in the invasion of Mato Grosso and the Battle of Riachuelo. In the latter, the vessel was rammed by the Brazilian frigate Amazonas and capsized, after which it was dismantled and set on fire.