Fructuoso Rivera had again become Uruguay's elected…

Fructuoso Rivera had again become Uruguay's elected president in March 1838.

In 1839 President Rivera, with the support of the French and of Argentine emigres, had issued a declaration of war against Argentina's dictator, Juan Manuel de Rosas, and had driven Rosas's forces from Uruguay.

The French, however, reach an agreement with Rosas and withdraw their troops from the Rio de la Plata region in 1840, leaving Montevideo vulnerable to Oribe's Argentine-backed forces.

For three years, the locus of the struggle is on Argentine territory.

Oribe and the Blancos ally themselves with Argentina's federalists, while Rivera and the Colorados side with Argentina's rival unitary forces, who favor the centralization of the Argentine state.

In 1842 Oribe defeats Rivera and later, on February 16, 1843, lays siege to Montevideo, at this time governed by the Colorados.