The Battle of Aljubarrota was a battle fought between the Kingdom of Portugal and the Crown of Castile on 14 August 1385. Aljubarrota is near Alcobaça.

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 place, 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. To celebrate his victory and acknowledge divine help, John I of Portugal ordered the construction of the monastery of Santa Maria da Vitória na Batalha and the founding of the town of Batalha (Portuguese for “battle”,Portuguese pronunciation: [bɐˈtaʎɐ]). The king, his wife Philippa of Lancaster, and several of his sons are buried in this monastery, today a UNESCO World Heritage Site.

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 decimating 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 child of his marriage with Leonor Telles de Meneses was a girl, Princess Beatrice of Portugal.

In April of that same year the King 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 discontent, and the Portuguese nobility was unwilling to support the claim of the princess because that could mean the incorporation of Portugal to Castile^[a]; also the powerful merchants of the capital, Lisbon, were enraged from being excluded from the negotiations. Without an undisputed option, Portugal remained without king from 1383–85, in an interregnum known as the 1383–85 Crisis.

The first clear act of hostility was taken 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 him 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 marked the first use of English defensive tactics on the Iberian peninsula, reportedly without any casualties to the Portuguese. A larger second expedition led by the Castilian king himself reached and besieged Lisbon for four months 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 an English force was to be sent to Portugal, to help defending the kingdom against its Castilian neighbor.^[2] 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 in 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 populations along the border, a common practice at the time and similar to what the English were doing in Scotland, 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.

English allies arrived on Easter of 1385, consisting of a company of about 100 English longbowmen, veterans from the Hundred Years’ War, sent to honor the Anglo-Portuguese Treaty of 1373 (presently the oldest active treaty in the world). The Portuguese set out to intercept the invading army near the town of Leiria. Nuno Álvares Pereira took 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 indicated for the chosen military tactic, a small flattened hill surrounded by creeks, with the very small settlement of Chão da Feira (Fair’s Ground) at its widest point, still present today.