In 1813 the new government in Buenos Aires convened a constituent assembly where Artigas emerged as a champion of federalism, demanding political and economic autonomy for each area, and for the Banda Oriental in particular. The assembly refused to seat the delegates from the Banda Oriental however, and Buenos Aires pursued a system based on unitary centralism.
As a result, Artigas broke with Buenos Aires and besieged Montevideo, taking the city in early 1815. Once the troops from Buenos Aires had withdrawn, the Banda Oriental appointed its first autonomous government. Artigas organized the Federal League under his protection, consisting of six provinces, four of which later became part of Argentina.
In 1816 a force of 10,000 Portuguese troops invaded the Banda Oriental from Brazil; they took Montevideo in January 1817. After nearly four more years of struggle Portuguese Brazil annexed the Banda Oriental as a province under the name of "Cisplatina". The Brazilian Empire became independent from Portugal in 1822. In response to the annexation, the Thirty-Three Orientals, led by Juan Antonio Lavalleja, declared independence on 25 August 1825 supported by the United Provinces of the Río de la Plata (present-day Argentina). This led to the 500-day-long Cisplatine War. Neither side gained the upper hand and in 1828 the Treaty of Montevideo, fostered by the United Kingdom, gave birth to Uruguay as an independent state. The nation's first constitution was adopted on 18 July 1830.
Blancos–Colorados conflicts
Further information: Uruguayan Civil War
The Colorados favored the exiled Argentinian liberal Unitarios, many of whom had taken refuge in Montevideo while the Blanco president Manuel Oribe was a close friend of the Argentinian ruler Manuel de Rosas. On 15 June 1838, an army led by the Colorado leader Rivera overthrew the president, who fled to Argentina. Rivera declared war on Rosas in 1839. The conflict would last thirteen years and become known as the Guerra Grande (the Great War).
In 1843, an Argentinian army overran Uruguay on Oribe's behalf but failed to take the capital. The siege of Montevideo, which began in February 1843, would last nine years. The besieged Uruguayans called on resident foreigners for help which led to a French and an Italian legion being formed, the latter led by the exiled Giuseppe Garibaldi.
In 1845, Britain and France intervened against Rosas to restore commerce to normal levels in the region. Their efforts proved ineffective and by 1849, tired of the war, both withdrew after signing a treaty favorable to Rosas. It appeared that Montevideo would finally fall when an uprising against Rosas, led by Justo José de Urquiza governor of Argentina's Entre Ríos Province began. The Brazilian intervention in May 1851 on behalf of the Colorados, combined with the uprising, changed the situation and Oribe was defeated. The siege of Montevideo was lifted and the Guerra Grande finally came to an end. Montevideo rewarded Brazil's support by signing treaties that confirmed Brazil's right to intervene in Uruguay's internal affairs.
In accordance with the 1851 treaties, Brazil intervened militarily in Uruguay as often as it deemed necessary. In 1865, the Triple Alliance was formed by the emperor of Brazil, the president of Argentina, and the Colorado general Venancio Flores, the Uruguayan head of government whom they both had helped to gain power. The Triple Alliance declared war on Paraguayan leader Francisco Solano López and the resulting Paraguayan War ended with the invasion of Paraguay and its defeat by the armies of the three countries. Montevideo, which was used as a supply station by the Brazilian navy, experienced a period of prosperity and relative calm during the war.
The constitutional government of General Lorenzo Batlle y Grau (1868–72) was forced to suppress an insurrection led by the National Party. After two years of struggle, a peace agreement was signed in 1872 that gave the Blancos a share in the emoluments and functions of government, through control of four of the departments of Uruguay. This establishment of the policy of co-participation represented the search for a new formula of compromise, based on the coexistence of the party in power and the party in opposition.
Between 1875 and 1886, the military became the center of power. During this authoritarian period, the government took steps toward the organization of the country as a modern state, encouraging its economic and social transformation. Pressure groups (consisting mainly of businessmen, hacendados, and industrialists) were organized and had a strong influence on government. A transition period (1886–90) followed, during which politicians began recovering lost ground and some civilian participation in government occurred.
