Task: Question Generation
Input (English): The first set of subsidiary forts were planned by the Portuguese at Montevideo in 1701 to establish a front line base to stop frequent insurrections by the Spaniards emanating from Buenos Aires. These fortifications were planned within the River Plate estuary at Colonia del Sacramento. However, this plan came to fruition only in November 1723, when Captain Manuel Henriques de Noronha reached the shores of Montevideo with soldiers, guns and colonists on his warship Nossa Senhora de Oliveara. They built a small square fortification. However, under siege from forces from Buenos Aires, the Portuguese withdrew from Montevideo Bay in January 1724, after signing an agreement with the Spaniards.
Output (English): the subsidiary forts were established to stop what?