[2] After achieving independence from the Iberian crowns in the early nineteenth century, Argentina and Brazil inherited a series of unresolved territorial disputes from their colonial powers.

There was competition on many levels, and their respective defense policies reflected mutual suspicion, but the Brazilian economic rise in the 1980s led to the accommodation of Argentina as a secondary regional power and increasing cooperation.

[6] Argentina and Brazil share the Río de la Plata basin– an area where Portuguese and Spanish conquistadors collided in their ambition to conquer new land for their respective crowns.

After achieving independence from the Iberian crowns in the early nineteenth century, the Argentine Republic and the Brazilian Empire inherited a series of unresolved territorial disputes from their colonial powers, involving Paraguay and Uruguay, the other two nations of the Río de la Plata basin.

From 1825 to 1828 the forces of the United Provinces of the Río de la Plata outfought those of the Brazilian Empire, until the signing of the Treaty of Montevideo that gave independence to Uruguay from both countries.

In contrast, the Argentine Republic's nineteenth century experience was marked by infighting between contending factions—those favoring a federalist republic—struggling against the strong centralist tendencies of the city of Buenos Aires (Unitarians).

Fear from possible Brazilian intervention in support for Chile amidst the War of the Pacific had led to Argentina maintaining status quo with Brazil.

In Brazil, the liberal revolution of 1930 overthrew the oligarchic coffee plantation owners and brought to power an urban middle class and business interests that promoted industrialization and modernization.

In 1966, Brazil and Paraguay concluded the Iguaçu Act, announcing their intention to build a Brazilian–Paraguayan hydroelectric plant, Itaipú dam, on the Paraná River, on the Argentina–Brazil–Paraguay border.

In October 1979, the Itaipú–Corpus Multilateral Treaty on Technical Cooperation was concluded, ending the dispute to the satisfaction of all three neighbors and opening the way for a dramatic improvement in relations.

[citation needed] Figueiredo, the last president of the military rulers who had governed Brazil for 21 years, visited Buenos Aires in May 1980 and signed, among other agreements, a series of accords to collaborate on nuclear issues.

[citation needed] Following the resolution of the water resources dispute and the Brazilian president's successful visit, an unexpected and traumatic event took place in Argentina that further improved bilateral relations: the 1982 Falklands War.

Brazil supported the Argentine claim over the Falkland Islands: After reviewing the issue regarding the Falkland Islands, His Excellency the President of the Federative Republic of Brazil expressed the support of his Government to the Argentine Republic, reaffirming his belief that the negotiations in progress will yield satisfactory results within a brief amount of time.However, when an RAF Vulcan Bomber was forced to land in Rio De Janeiro after attacking Stanley Airport, the Brazilian government refused to hand over crew as prisoners of war to Argentina and instead released them after a brief period of internment.

[citation needed] After hostilities ended in June 1982, Buenos Aires chose Brazil to represent its interests in London until full diplomatic relations with United Kingdom were restored in 1990.

Thus, despite rivalry and historical suspicions, Brazil's actions and policies during the most traumatic period of Argentina's recent history—objectionable military rule, near-conflict with Chile and the Falklands War—were fundamental to build trust between the two countries.

Among his main achievements, President Alfonsín started to resolve the enduring territorial conflict with Chile during his six-year term, and significantly improved relations with Brazil.

While still under military rule, Brazil initiated a policy of improving relations with its South American neighbors, and Argentina was considered the key country in this effort.

In accordance with the friendship policy, both armies dissolved or moved major units previously located at their common border (e.g. Argentine's 7th Jungle and 3rd Motorized Infantry Brigades).

[11] This was met with reciprocity in Brazil, as President Luiz Inácio Lula da Silva placed Argentina as the main priority of his foreign policy.

[12] From the Brazilian perspective, only with this strategic alliance would it be possible to transform South America into a world power bloc, one of the goals of Lula da Silva's foreign policy.

For me the main idea is that of a strategic relationship with Argentina, which should shine itself in all areas of interest of both countries", said Rousseff in conversations with local newspapers before arriving in Buenos Aires.

Proof of the difficult relationship was shown during the 2019 Argentine general election, in which Bolsonaro showed all his support for his right-wing ally Mauricio Macri.

It showed the turbulent relationship between the two South American powers, where for the first time in 23 years there would be no Brazilian representation in the assumption of their main commercial ally, putting at risk the continuity of the organization of Mercosur, already almost non-existent due to the suspension of Venezuela during Maduro's dictatorship.

[27] In July 2020, the Mercosur summit began via videoconference, where the coldness of both presidents was shown; they did not speak each other and Férnandez made an indirect attack addressing Bolsonaro.

During President Lula's state visit to Buenos Aires on 22 February 2008, the two countries established a binational commission on pursuing joint uranium enrichment for nuclear energy purposes.

[39] Brazil has criticized the UN over its stance on the dispute,[40] and accused the United Kingdom of using its status as permanent member of the UN's Security Council to prevent the debate from being reopened.

[5] Argentina and Brazil both hold membership in a number of multinational organizations, including: Since 2003, Presidential meetings are held every six months alternately in each country,[50] and besides those there are more for other reasons (UNASUR, Mercosur, G20, etc.