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The Public Forces (Portuguese: Forças Públicas) of the states of Brazil were already called "small state armies" in the First Brazilian Republic (1889–1930) due to their martial character. They took part in the various struggles and rebellions of the period alongside, and sometimes against, the Brazilian Army. Their character was hybrid, police and warfare. They emerged in the federalism of the First Republic as shields of state power against central power, represented by the Army, and were dismantled by the federal government in the Vargas Era (1930–1945) onwards, losing their conventional warfare capabilities.
The Brazilian Empire already had militarized police forces, but its provinces were not autonomous. Only in the Republic did state presidents (governors) need military forces in their relations with each other and with the Union. By preventing federal intervention and securing the authority of state oligarchies, they strengthened the First Republic's political system. By 1920, half the states had militias larger than the federal army garrisons. The three most important, São Paulo, Minas Gerais and Rio Grande do Sul, had the strongest "small armies". The largest, the Public Force of São Paulo, was prestigious; it hired a French training mission years before the Brazilian Army and had artillery and aviation. The poorer states had modest forces. The federal army, in turn, was still small and weak at the turn of the century. The existence of Public Forces, National Guard and "patriotic battalions" meant the federal army was not the only land military force, a situation condemned by many of its officers. In case of a foreign war the Public Forces would increase Brazilian power, but they could also obstruct Brazil's international power projection.
After the 1930 Revolution and especially in the Estado Novo (1937–1945), Getúlio Vargas promoted political centralization and the Army realized its ambition of hegemony over the security forces. Central power controlled state forces and expropriated their heavy weapons. The new role of the Military Police, as the Public Forces became known, was that of auxiliary and reserve forces for the Army. Even after 1945, when centralization was not so great, their focus gradually shifted from conventional warfare to public order. At the time of the 1961 Legality Campaign and the 1964 coup d'état, they still had a bellicose character. Several, notably the Military Brigade of Rio Grande do Sul and the Military Police of Minas Gerais, prepared for combat, which did not occur, against the Armed Forces, which now had much greater firepower. The Brazilian military dictatorship (1964–1985) confirmed the Army's control over the police.