In the spring of 1147, the Pope authorized the crusade in the Iberian peninsula. He also authorized Alfonso VII of León and Castile to equate his campaigns against the Moors with the rest of the Second Crusade. In May 1147, a contingent of crusaders left from Dartmouth in England. They had intended to sail directly to the Holy Land, but weather forced the ships to stop on the Portuguese coast, at the northern city of Porto on 16 June 1147. There they were convinced to meet with King Afonso I of Portugal. The crusaders agreed to help the King attack Lisbon, with a solemn agreement that offered to the crusaders the pillage of the city's goods and the ransom money for expected prisoners.

The siege of Lisbon, from 1 July to 25 October 1147, was the military action that brought the city of Lisbon under definitive Portuguese control and expelled its Moorish overlords. It is seen as a pivotal battle of the wider Reconquista.