Though its participants originally justified their actions as an effort to protect Brazil's constitution, the military coup of March 31, 1964 in fact inaugurated a new era in Brazilian politics. The new government, first run by a military junta, proceeded to concentrate power in the hands of the Brazilian Armed Forces, with Brazil's congress naming Field Marshal Humberto de Alencar Castelo Branco to the presidency on April 11, 1964. Along with this concentration of power came the alteration and suspension of the political and legal rights of citizens and elected officials over the following decade. This began on April 9, 1964 with the declaration of Institutional Act n.1, the first in a series of Institutional Acts External or proclamations made under the presidencies of General Humberto de Alencar Castelo Branco, General Artur da Costa e Silva, the Military Junta of 1969, and General Emílio Garrastazu Médici. While the dictatorship significantly reduced everyday political rights compared to the earlier democratic era, the military government continued to employ nationalist policies focused on the country's economic development and a foreign policy independent from the interests of the Cold War superpowers.

US-Brazil relations waxed and waned in this period. Under the dictatorship, the Brazilian military’s strong anti-communist position converged with that of the United States. As a result, US foreign aid to Brazil reached new heights. From 1964 to 1970, Brazil received more money than any other country in South America from institutions such as USAID and the World Bank. American businesses also increased their investments in Brazil. However, in the 1970s, these close ties eroded. US representatives’ denunciation of the Brazilian military government's human rights abuses, diverging positions on the independence struggles of Asia and Africa, increasing antagonism regarding nuclear non-proliferation and Brazil's exchange of nuclear technologies and raw materials with Germany, and Brazilian relations with Central America and Cuba all gradually distanced the governments of the United States and Brazil during this period. By the 1980s, these issues strained the "special relationship" between the two countries which had endured since the late nineteenth century.

Even as many of the military government’s actions during the dictatorship distanced political ties with the government of the United States, resistance to the dictatorship's oppression served to approximate the citizens and civil groups of each country. Opponents of Brazil's military dictatorship, including religious groups, artists, political activists, and academics, pressured the United States to change its foreign policy towards Brazil's military regime and championed the cause of democracy in Brazil. Beginning in 1974, the Brazilian military government under President General Ernesto Geisel initiated the gradual process of redemocratization (called abertura or "opening" in Portuguese). Promising a slow return to democratic rule, this process continued under President General João Figueiredo. By 1985, abertura resulted in popular elections and the end of the military dictatorship.

Key figures

• Castelo Branco, Humberto de Alencar, 1900-1967
• Figueiredo, João Baptista de Oliveira, 1918-1999
• Geisel, Ernesto
• Médici, Emílio Garrastazu, 1905-1985

Key subjects

• Brazil--Politics and government--1964-1985
• Dictatorship--Brazil--History--20th century
• Nuclear industry--Government policy--Brazil