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.

The political system in the First Brazilian Republic left extensive powers in the hands of state oligarchies and municipal coronelism. The power of São Paulo and Minas Gerais, the milk coffee policy, was preponderant. The 1891 Constitution had federalism as its principle and allowed states to negotiate with each other and to legislate on any matter that had not been denied them—including the organization of military forces. Even in the 1920s, there was still no national consciousness, and state identities were strong.

The Army, one of the armed branches of the central power, was on the verge of collapse in the late 1890s. It was small, unprepared and of little operational capacity. It was divorced from the civilian elites, especially in São Paulo and Minas Gerais, and was not yet a "national organization capable of effectively planning and executing a defense policy in its broadest sense". It gradually approached this ambition over the period. The political system left a secondary role to the Army, which should solve what the local forces could not handle. There was still theoretically the National Guard, and in times of crisis local colonels mobilized "patriotic battalions" with their peons and henchmen. Thus, there were armed troops other than the Army.

Since the Empire, police had already served as a kind of Army reserve, participating in the Paraguayan War and internal conflicts. However, the imperial provinces were not autonomous and did not need military force to relate to each other and to the central power. Policing was local and largely the responsibility of the National Guard. Only in the Republic did the presidents of the states (governors) build their small armies, capable of rivaling the federal army, in the midst of tensions with the Union.

At the behest of state oligarchies, the police maintained state influence in national politics, prevented federal intervention, and preserved the governors' policy [pt] and the political system of the First Republic. Since the Empire they already had the rigor of hierarchy and discipline, becoming even more militarized in the Republic. Even so, public safety was still one of their duties, and the character of the force was hybrid, both military and police. Within the states, the Public Forces served as praetorian guards for the groups in power and could be used against internal enemies.

In 1909, Albuquerque Lins [pt], then president of São Paulo, defined his Public Force as a "small army of São Paulo", and before him other observers already noticed the phenomenon. Journalist João Camilo de Oliveira Torres wrote of the existence of "a National Army and two dozen state armies". Another term used was "state militia". The official denomination of "Public Force" was not standard in all states and times, and in Rio Grande do Sul the name "Military Brigade" remained.