The well fortified city of Alcácer do Sal was a frontier outpost of the Almohad Caliphate facing Portugal.

The expedition was the brainchild of Bishop Soeiro II of Lisbon, whose diocese was threatened by regular raids from Alcácer.

King Afonso II of Portugal did not take part in person, but the city was incorporated into his kingdom after its capitulation.

The crusaders who took part in the siege, mainly from the Rhineland and the Low Countries, did so without papal authorization and were afterwards ordered to continue on to the Holy Land.

[1] The Carmen de expugnatione Salaciae was written by Goswin of Bossut, a priest from the Duchy of Brabant, for Bishop Soeiro.

[2] The De itinere Frisonum is a contemporary record of the Frisian crusaders who sailed with the Germans to Portugal but did not partake in the siege of Alcácer.

[8] In the late 9th century, Alcácer do Sal (Qaṣr Abī Dānis in Arabic) was constructed at the mouth of the Sado by the Umayyads as a fortress against the Vikings.

[19][21] According to the Gesta, which mentions both counts, William was elected sole leader at Dartmouth when the crusaders "decided under him on laws and new rules concerning the observance of peace.

The army marched overland to rendezvous with the fleet, which left Lisbon at the end of July and sailed up the Sado.

[24] The Carmen describes poetically how the attackers attempted to fill in the moat with fig and olive trees, but the defenders set the infill on fire.

[27] According to the Rawḍ al-Qirṭās, the Caliph Yūsuf II ordered the governors of Córdoba, Jaén, Seville and other places in the Gharb al-Andalus to raise a relief army.

[26] As the Muslim relief army arrived, Christian reinforcements, raised by several Portuguese and Leonese barons, were on the march.

The Templars were led by Pedro Álvarez de Alvito, master of the order in Spain; the knights of Santiago by Martim Barregão, commander of Palmela; and the Hospitallers by the prior of Portugal.

[32] In addition, Caesarius of Heisterbach reports that an eyewitness told him how "the galleys which [the Saracens] had brought over the sea against the Christians were put to flight by the terror of [a] celestial vision" of "a whiteclad host, wearing red crosses upon the breast.

[31] After the victory, the bishops of Lisbon and Évora and the leaders of the three orders who were present wrote to Honorius III with three requests: that the pope order the crusaders to remain in Portugal for one year for mopping up operations; that the crusade indulgence be extended to those who took part and to those who would take part in Portuguese operations in the future; and that the Iberian proceeds of the tax of a twentieth (vicesima) levied on ecclesiastical incomes by Innocent III in the bull Ad liberandum (1215) for the Holy Land be set aside for operations in Iberia.

These requests were based on the proviso in the bull Quia maior (1213) that "if perchance it were needed, we [the pope] would take care to give our attention to any serious situation that arises.

"[6] The clergy also reported to Honorius three miracles that vindicated the operation: the timely arrival of the reinforcements and the appearance in the sky of a cross and a heavenly army.

[33] At the same time, William of Holland wrote to the pope to ask whether he should continue to lead his army to the Holy Land or remain to fight in Portugal.

[24] Honorius did acknowledge the triumph at Alcácer do Sal when, on 11 January 1218, he reissued the bull Manifestis probatum confirming the independence of Portugal, wherein he attributed the victory to Afonso II.