By 1759, all captaincies had been returned to the Crown, with captains becoming appointed rather than recognized by inheritance. Some captaincies were designated as captaincies-general, to which other captaincies were subordinated. In addition, the State of Grão-Pará and Rio Negro and the State of Maranhão and Piauí, which had been split from the State of Maranhão, were reincorporated into the State of Brazil in 1775, under a single governor-general. This centralization later helped to keep Brazil as a unified nation-state, avoiding fragmentation similar to that of the Spanish domains. The captaincies became provinces in 1821, during the final years of the Kingdom of Brazil (united with Portugal), and maintained that designation after independence in 1822 under the Empire of Brazil. Most internal boundaries were kept unchanged from the end of the colonial period, generally following natural features such as rivers and mountain ridges. Some changes were made to suit domestic politics (transferring the Triângulo Mineiro from Goiás to Minas Gerais, transferring the south bank of the São Francisco River from Pernambuco to Minas Gerais and later to Bahia, separating the capital city of Rio de Janeiro as a Neutral Municipality outside of any province, splitting Amazonas from Pará, and splitting Paraná from São Paulo), as well as international border adjustments resulting from diplomatic settlement of territorial disputes. The Cisplatine Province was annexed into Brazil in 1821, declared independence as Uruguay in 1825, and was recognized by the Treaty of Montevideo in 1828. When Brazil became a republic in 1889, all provinces became states, and the Neutral Municipality became the Federal District. In 1903, Brazil acquired the territory of Acre from Bolivia with the Treaty of Petrópolis. In 1942–1943, with the entrance of Brazil into World War II, the Vargas regime detached six strategic territories from the borders of the country in order to administer them directly: the archipelago of Fernando de Noronha (from Pernambuco), Amapá (from Pará), Rio Branco (from Amazonas), Guaporé (from Mato Grosso and Amazonas), Ponta Porã (from Mato Grosso) and Iguaçu (from Paraná and Santa Catarina). Shortly after the war, the Brazilian constitution of 1946 returned Ponta Porã and Iguaçu to their original states.