A force of ten thousand Portuguese troops invade the Banda Oriental from Brazil in 1816; they take Montevideo in January 1817.

After nearly four more years of struggle, the Portuguese Kingdom of Brazil annexes the Banda Oriental as a province under the name of "Cisplatina".

The Brazilian Empire becomes independent of Portugal in 1822.

In response to the annexation, the Thirty-Three Orientals, led by Juan Antonio Lavalleja, declare independence on August 25, 1825, supported by the United Provinces of the Río de la Plata (present-day Argentina).

This leads to the five hundred-day-long Cisplatine War.

Neither side gains the upper hand and in 1828 the Treaty of Montevideo, fostered by the United Kingdom through the diplomatic efforts of Viscount John Ponsonby, will give birth to Uruguay as an independent state.