The Empire of Brazil, Argentina, and Uruguay sign the Treaty of the Triple Alliance.

The Empire of Brazil was a 19th-century state that broadly comprised the territories which form modern Brazil and Uruguay until the latter achieved independence in 1828. The empire's government was a representative parliamentary constitutional monarchy under the rule of Emperors Pedro I and his son Pedro II. A colony of the Kingdom of Portugal, Brazil became the seat of the Portuguese Empire in 1808, when the Portuguese Prince regent, later King Dom John VI, fled from Napoleon's invasion of Portugal and established himself and his government in the Brazilian city of Rio de Janeiro. John VI later returned to Portugal, leaving his eldest son and heir-apparent, Pedro, to rule the Kingdom of Brazil as regent. On 7 September 1822, Pedro declared the independence of Brazil and, after waging a successful war against his father's kingdom, was acclaimed on 12 October as Pedro I, the first Emperor of Brazil. The new country was huge, sparsely populated, and ethnically diverse.

Argentina, officially the Argentine Republic, is a country in the southern half of South America. It covers an area of 2,780,085 km² (1,073,397 sq mi), making it the second-largest country in South America after Brazil, the fourth-largest country in the Americas, and the eighth-largest country in the world. Argentina shares the bulk of the Southern Cone with Chile to the west, and is also bordered by Bolivia and Paraguay to the north, Brazil to the northeast, Uruguay and the South Atlantic Ocean to the east, and the Drake Passage to the south. Argentina is a federal state subdivided into twenty-three provinces, and one autonomous city, which is the federal capital and largest city of the nation, Buenos Aires. The provinces and the capital have their own constitutions, but exist under a federal system. Argentina claims sovereignty over the Falkland Islands, South Georgia and the South Sandwich Islands, the Southern Patagonian Ice Field, and a part of Antarctica.

Uruguay, officially the Oriental Republic of Uruguay, is a country in South America. It shares borders with Argentina to its west and southwest and Brazil to its north and northeast, while bordering the Río de la Plata to the south and the Atlantic Ocean to the southeast. It is part of the Southern Cone region of South America. Uruguay covers an area of approximately 176,215 square kilometers (68,037 sq mi). It has a population of almost 3.5 million people, of whom nearly 2 million live in the metropolitan area of its capital and largest city, Montevideo.