Spain, aided by the French, launched an invasion of Portugal and succeeded in capturing Almeida. The arrival of British reinforcements stalled a further Spanish advance, and in the Battle of Valencia de Alcántara British-Portuguese forces overran a major Spanish supply base. The invaders were stopped on the heights in front of Abrantes (called the pass to Lisbon) where the Anglo-Portuguese were entrenched. Eventually the Anglo-Portuguese army, aided by guerrillas and practicing a scorched earth strategy, chased the greatly reduced Franco-Spanish army back to Spain, recovering almost all the lost towns, among them the Spanish headquarters in Castelo Branco full of wounded and sick that had been left behind.

The Franco-Spanish army (which had their supply lines from Spain cut off by the guerrillas) was virtually destroyed by a deadly scorched earth strategy. Peasants abandoned all nearby villages and took with them or destroyed the crops, food and all else that could be used by the invaders, including the roads and houses.