General José de San Martín, organized the armies that defeated the royalists and contributed decively to the independence of Chile and Peru. 

   Peace was restored in 1820, but the central issue, formation of a stable government, remained unresolved. Throughout most of the following decade a state of anarchy, further compounded by war with Brazil from 1825 to 1827, prevailed in the United Provinces. Brazil was defeated in the conflict, a result of rival claims to Uruguay, which emerged as an independent state. 

The national political turmoil lessened appreciably after the 1829 election of General Juan Manuel de Rosas as governor of Buenos Aires Province. A federalist, Rosas cemented friendly relations with other provinces, thereby winning broad popular support. He rapidly extended his authority over the United Provinces which became known as the Argentine Confederation, and during his rule all opposition groups were crushed or driven underground. 

In 1833, Great Britain occupied the Malvinas Islands (Falkland Islands). 

The dictatorial regime of Rosas was overthrown in 1852 by a revolutionary group led by General Justo Urquiza, a former governor of Entre Ríos Province, who received assistance from Uruguay and Brazil. In 1853 a federal constitution was adopted, and Urquiza became first president of the Argentine Republic. Buenos Aires Province, refusing to adhere to the new constitution, proclaimed independence in 1854. The mutual hostility of the two states flared into war in 1859. The Argentine Republic won a quick victory in this conflict, and in October 1859, Buenos Aires agreed to join the federation.

The province was, however, the center of another rebellion against the central government in 1861. Headed by General Bartolome Mitre, the rebels defeated the national army in September of that year. The president of the republic resigned on November 5. In May of the next year a national convention elected Mitre to the presidency and designated the city of Buenos Aires as the national capital. With these events, Buenos Aires Province, the wealthiest and most populous in the union, achieved temporary control over the remainder of the nation. 

Turmoil in Uruguay brought on a Paraguayan invasion of Argentine territory in 1865, beginning the bloody War of the Triple Alliance, which ended in complete victory for Argentina, Brazil, and Uruguay in 1870. During the next decade the conquest of the Pampas as far as the Río Negro was completed, and the threat of hostile Native Americans from that direction was eliminated. This so-called War of the Desert (1879-1880), directed by General Julio A. Roca, opened up vast new areas for grazing and farming. In 1880 Roca, who opposed the ascendency of Buenos Aires in national affairs, was elected to the presidency. In the aftermath of his victory, the city of Buenos Aires was separated from the province and established as a federal district and national capital. A long-standing boundary dispute with Chile was settled in 1881; through this agreement Argentina acquired the title to the eastern half of Tierra del Fuego. In 1895 a boundary dispute with Brazil was submitted to arbitration by the United States, which awarded about 65,000 sq km (about 25,000 sq mi) of territory to Argentina. The country became involved in a serious controversy with Chile regarding the Patagonian frontier in 1899. This dispute was finally settled in 1902, with Great Britain acting as arbitrator. 

In the half century following 1880, Argentina made remarkable economic and social progress. During the first decade of the 20th century the country emerged as one of the leading nations of South America.