Born at Alagoas, Fonseca made a military career, putting down the Praieira revolt in Pernambuco, in 1848, which was Brazil's response to the European year of failed liberal revoltions. He saw action in the War of Cisplatina and in Brazil's war with Paraguay (1864 - 1870), attaining a rank of captain, later (1884) raised to field-marshal, then full marshal. His personal courage, military competence and manly style made him a national figure.
As Governor of Rio Grande do Sul, Fonseca was courted by republican intellectuals such as Benjamin Constant and Rui Barbosa in the café society of São Paolo. In 1886, alerted that the imperial government was ordering the arrest of prominent republicans, Fonseca went to Rio de Janeiro and assumed leadership of the army faction that was favorable to the abolition of slavery. His prestige placed him at the head of the military coup that deposed the emperor, November 15, 1889, and he was briefly the head of the provisional government that called a Constituent Congress to draft a new constitution for a United States of Brazil. Soon, however, he was in conflict with the civilian republican leaders.
His election as president, February 26, 1891, by a narrow plurality, was backed by military pressure on Congress. A few days later he took the oath of office at Quinta de Boa Vista, the imperial, now presidential palace in Rio de Janiero.
The Fonseca government, divided by political and personal animosity between the president and vice president Floriano Peixoto, encountered strong opposition within the Congress, which chose a policy of obstruction. Arbitrary presidential decrees, including concession of the port of Torres to a private company, a stroke of corporate nationalism in the style of Napoleon III, fixed the resistance in Congress, which coalesced round vice-president Peixoto, and soured public opinion. The situation approached a climax when Fonseca dissolved the National Congress and declared a 'state of emergency,' November 3, 1891. A group of deputies opposed this decision and found support among the high-ranked officers of the Navy including Admiral Custódio José de Melo. The marshal found himself on the brink of a civil war. On 23 Nov 1891 he signed a resignation (to no one in particular) and turned over the presidency to Floriano Peixoto.
Deodoro da Fonseca died in Rio de Janiero on August 23, 1892.
See also: List of Presidents of Brazil