In the midst of the 1864-1870 Paraguayan War, the Paraguayan Navy launched a bold assault on the larger Brazilian Navy while it lay at anchor on the Riachuelo River. On the morning of June 11, 1865 a Paraguayan force of nine ships and seven armed barges approached the slumbering Brazilian fleet. The original plan was to board the Brazilian vessels while their crews were still ashore asleep, however, at the last minute the Paraguayan commander deviated from the plan and opened fire on the Brazilian ships from a distance. This proved to be a devastating mistake as the Brazilians were able to quickly rally, board their vessels and bring their superior firepower to bear on the smaller Paraguayan force.
The Paraguayan fleet positioned itself so as to trap the Brazilian fleet in the river. The move proved pointless, though, as the superior Brazilian fleet audaciously steamed at the Paraguayans and rammed and sank several of the Paraguay vessels. By the time the smoke cleared, the Brazilians had lost only a single vessel while sinking four of the nine Paraguayan ships and all seven of their armed barges. The victory helped the Brazilians and their allies turn the tide of the war as Paraguay had enjoyed multiple victories early in the war. In all, nearly 1,000 souls perished in the early morning battle.