José Fructuoso Rivera (c1790-1854) again became the elected president [of Uruguay] in March 1838. In 1839 President Rivera, with the support of the French and of Argentine émigrés, issued a declaration of war against Argentina's dictator, Juan Manuel de Rosas, and drove Rosas's forces from Uruguay. The French, however, reached an agreement with Rosas and withdrew their troops from the Río de la Plata region in 1840, leaving Montevideo vulnerable to the Blancos, led by Manuel Oribe, and their Argentine allies. For three years, the locus of the struggle was on Argentine territory. Oribe and the Blancos allied themselves with Argentina's federalists, while Rivera and the Colorados sided with Argentina's rival unitary forces, who favored the centralization of the Argentine state. In 1842 Oribe defeated Rivera and later, on February 16, 1843, laid siege to Montevideo, then governed by the Colorados. The siege continued for eight years (1843-1851). The Italian patriot Giuseppe Garibaldi (1807-82), in exile in Uruguay, was one of the defenders of Montevideo. British and French naval forces arrived in the area, occupied parts of Uruguay and Martin Garcia Island at the mouth of the Uruguay River, and blockaded the Rio de la Plata (1845-49). Oribe was forced to abandon the siege of Montevideo and sign a treaty (1851) allowing the Colorados to govern Uruguay, as a consequence of the war against the Rosas dictatorship in Argentina 1851-1852.