 In 1461, Navarre was invaded by Castilian troops, resulting in the territorial loss of the Sonsierra, as well as several strongholds, to the Castilians.Starting in 1474, Ferdinand II of Aragon, instituted a combination of alliances and military efforts aimed at securing the control of neighbouring kingdoms.This included turning Navarre into a "de facto" protectorate of Castile in 1476.