The Battle of Aljubarrota occurred during the Portuguese Crisis of 1383-85, a conflict rooted in succession disputes following the death of King Fernando I of Portugal. The death led to a struggle for the throne between the supporters of his daughter Beatriz, who was married to John I of Castile, and those who backed John I of Portugal, a master of Aviz, who declared himself king. Amidst rising tensions, the Castilian forces invaded Portugal, leading to the decisive engagement at Aljubarrota on August 14, 1385.

Portuguese forces, numbering around 6,000, were commanded strategically by John I, aided by his half-brother, the Master of Aviz, and supported by notable figures such as Nuno Álvares Pereira. They utilized the uneven terrain to their advantage, employing tactics that included concealed positions and disciplined infantry formations. Castilian forces, comprising over 25,000 troops, underestimated the resolve and tactical acumen of the Portuguese army.

The battle unfolded with a ferocious clash of arms, highlighting the tactical proficiency and motivations of Portuguese soldiers, who were fighting for their country's autonomy. The Portuguese forces emerged victorious due to surprise and efficient combat strategies, inflicting heavy casualties on the Castilians and forcing them to retreat. This victory solidified John I's reign and affirmed Portugal's independence, effectively ending the threats posed by Castile, which had significant implications for the political landscape of the Iberian Peninsula.