information:classification, structural:reformatting
Brazil and its allies

At the beginning of the war, the military forces of Brazil, Argentina, and Uruguay were far smaller than Paraguay's. Argentina had approximately 8,500 regular troops and a naval squadron of four steamers and one schooner. Uruguay entered the war with fewer than 2,000 men and no navy. Many of Brazil's 16,000 troops were located in its southern garrisons. The Brazilian advantage, though, was in its navy, comprising 45 ships with 239 cannons and about 4,000 well-trained crew. A great part of the squadron was already in the Rio de la Plata basin, where it had acted under the Marquis of Tamandaré in the intervention against Aguirre's government.

Brazil, however, was unprepared to fight a war. Its army was disorganized. The troops it used in Uruguay were mostly armed contingents of gauchos and the National Guard. While some Brazilian accounts of the war described their infantry as volunteers (Voluntários da Pátria), other Argentine revisionist and Paraguayan accounts disparaged the Brazilian infantry as mainly recruited from slaves and the landless (largely black) underclass, who were promised free land for enlisting. The cavalry was formed from the National Guard of Rio Grande do Sul.

Ultimately, a total of about 146,000 Brazilians fought in the war from 1864 to 1870, consisting of the 10,025 army soldiers stationed in Uruguayan territory in 1864, 2,047 that were in the province of Mato Grosso, 55,985 Fatherland Volunteers, 60,009 National Guardsmen, 8,570 ex-slaves who had been freed to be sent to war, and 9,177 navy personnel. Another 18,000 National Guard troops stayed behind to defend Brazilian territory.