Map - Battle of Aljubarrota (Centro de Interpretação da Batalha de Aljubarrota)

The Battle of Aljubarrota (see Aljubarrota) was fought between the Kingdom of Portugal and the Crown of Castile on 14 August 1385. Forces commanded by King John I of Portugal and his general Nuno Álvares Pereira, with the support of English allies, opposed the army of King John I of Castile with its Aragonese, Italian and French allies at São Jorge, between the towns of Leiria and Alcobaça, in central Portugal. The result was a decisive victory for the Portuguese, ruling out Castilian ambitions to the Portuguese throne, ending the 1383–85 Crisis and assuring John as King of Portugal.

Portuguese independence was confirmed and a new dynasty, the House of Aviz, was established. Scattered border confrontations with Castilian troops would persist until the death of John I of Castile in 1390, but these posed no real threat to the new dynasty.

The end of the 14th century in Europe was a time of revolution and crisis, with the Hundred Years' War between the English and the French for Western France, the Black Death devastating the continent, and famine afflicting the poor. Portugal was no exception. In October 1383, King Ferdinand I of Portugal died with no son to inherit the crown. The only surviving child of his marriage with Leonor Telles de Meneses was a girl, Princess Beatrice of Portugal.

In April of that same year the King had signed the Treaty of Salvaterra de Magos with King Juan I of Castile. The treaty determined that Princess Beatrice was to marry Juan I, king of Castile, and the Crown of Portugal would belong to the descendants of this union. This situation left the majority of the Portuguese discontented, and the Portuguese nobility was unwilling to support the claim of the princess because that could mean the incorporation of Portugal into Castile. Also, the powerful merchants of the capital, Lisbon, were enraged at being excluded from the negotiations. Without an undisputed option, Portugal remained without a king from 1383–85, in an interregnum known as the 1383–85 Crisis.

The first clear act of hostility was carried out in December 1383 by the faction of John (João), the Grand Master of the Aviz Order (and a natural son of Peter I of Portugal), with the murder of Count Andeiro. This prompted the Lisbon merchants to name him "rector and defender of the realm". However, the Castilian king would not relinquish his and his wife's claims to the throne. In an effort to normalize the situation and secure the crown for himself or Beatrice, he forced Leonor to abdicate from the regency. In April 1384, in Alentejo, a punitive expedition was promptly defeated by Nuno Álvares Pereira, leading a much smaller Portuguese army at the Battle of Atoleiros. This was an example of the use of the defensive tactic of forming an infantry square to repel cavalry, reportedly without any casualties to the Portuguese. A larger second expedition led by the Castilian king himself reached and besieged Lisbon for four months in the summer of 1384, before being forced to retreat by a shortage of food supplies due to harassment from Nuno Álvares Pereira, and the bubonic plague.

In order to secure his claim, John of Aviz engaged in politics and intense diplomatic negotiations with both the Holy See and England. In October 1384, Richard II wrote to John (later King John I), regent of Portugal, reporting on negotiations, conducted in England, with John's envoys - Dom Fernando, master of the order of Santiago, and Laurence Fogaça, chancellor of Portugal saying that an agreement had been reached under which a small English contingent was to be sent to Portugal, to help defend the kingdom against its Castilian neighbor. On 6 April 1385, (the anniversary of the "miraculous" battle of Atoleiros, a fortuitous date), the Council of the kingdom (Cortes in Portuguese) assembled in Coimbra and declared him King John I of Portugal. After his accession to the throne, John I of Portugal proceeded to annex the cities whose military commanders supported Princess Beatrice and her husband's claims, namely Caminha, Braga, and Guimarães among others.

Enraged by this "rebellion", Juan I ordered a host of 31,000 men to engage in a two-pronged invasion in May. The smaller Northern force sacked and burnt towns along the border, before being defeated by local Portuguese nobles in the battle of Trancoso, in the first week of June. On the news of the invasion by the Castilians, John I of Portugal's army met with Nuno Álvares Pereira, the Constable of Portugal, in the town of Tomar. There they decided to face the Castilians before they could get close to Lisbon and lay siege to it again.

Mercenaries arrived from Gascony at Easter of 1385, sent to honor the Anglo-Portuguese Treaty of 1373 (still the oldest active international treaty in the world). This company was composed of about 200 English longbowmen, veterans from the Hundred Years' War, and around 500 locally recruited men-at-arms mostly of English and Gascon background, though a Florentine volunteer was also mentioned.

The Portuguese set out to intercept the invading army near the town of Leiria. Nuno Álvares Pereira took on the task of choosing the ground for the battle. Russell notes that the two Portuguese leaders (Nuno Álvares and Antão Vasques) had already shown themselves masters of the new developments in methods of warfare, i.e. the use of archers and dismounted men-at-arms. The chosen location was São Jorge near Aljubarrota, especially suitable for the chosen military tactic, being a small flattened hill surrounded by creeks, with the very small settlement of Chão da Feira at its widest point, still present today.

By having his army on the road to Lisbon, John de Avis effectively lured John of Castille's attention from laying siege at the capital itself and morally forced him to offer battle there, on his own terms. As in the Battle of Agincourt, winning this pitched battle would mean a decisive victory for Avis' cause, even with smaller numbers and resources.