In 1924, still as lieutenant, he completed the Infantry Advanced Course and, upon returning to the 12th RI, was assigned the task of commanding a detachment from the unit and integrating the legal forces that would come to grips with and overcome internal revolts hatched in São Paulo in 1925.

Section (Operations) of the Brazilian Expeditionary Force (FEB) during World War II, in Italy, remaining for three hundred days on the battlefields.

According to Marshal Cordeiro de Farias, Castello won exceptional prestige at FEB, for being a great strategist and having a privileged head.

Promoted to Colonel in 1945, Castelo Branco returned to Brazil with the firm intention of transmitting his professional experiences to the officers of the Army.

Months later, when trade union organizations resolved to hand over to the Minister a golden sword, Castelo severely broke with General Lott.

Castelo Branco was vested with emergency powers under the First Institutional Act, which among other things allowed him to cancel the political rights of "subversive elements" for ten years.

[6][7] However, the hard-line officers within the regime (known as linha-dura) with the support of War Minister Artur da Costa e Silva, wanted to stay in power for a greater period of time in order to achieve their political goals.

[6][7] Events reached a breaking point in October 1965, when opposition candidates won the governorships of the major states of Minas Gerais and Guanabara.

Another coup was averted after Costa e Silva persuaded hard-liners to recognize the election results in return for Castelo Branco's promise to implement a tougher policy.

Castelo Branco's government, unlike previous directly elected Presidents Juscelino Kubitschek, Jânio Quadros and João Goulart, was bankrolled from the start by the credits and loans from World Bank, International Monetary Fund and massive investment from multinational American companies, which saw the Brazilian right-wing military dictatorship as a new, economically stable Western ally against international communism in Latin America during the Cold War.