John I of Portugal

John I of Portugal
April 11, 1357 – August 14, 1433

John I of Portugal was born on April 11, 1357, to King Peter I of Portugal and his mistress, Teresa Lourenco. As an illegitimate son, John wasn’t intended to inherit the throne. Instead, Peter appointed the 6-year-old as the master of the Order of Aviz, a military organization. In 1367, Peter died, and his legitimate son, Ferdinand I, succeeded him. During his reign, the king had a tumultuous relationship with two Castilian kings: Henry II and Juan I. As part of a peace treaty, Ferdinand agreed to have his only child, Beatriz, marry Juan.

Because of the agreement, Portugal’s independence would be compromised after Ferdinand I’s death. When the king died in October 1383, his widow, Queen Leanor, recognized Juan I as Ferdinand’s successor. However, the queen’s decision proved to be widely unpopular. In response, a nationalist group formed to oppose her.

Leonor distrusted John. In 1382, she persuaded Ferdinand I to imprison his half-brother on false charges of treason. Although John was innocent, he came dangerously close to being executed. After the king’s death, John felt threatened by Leonor’s appointment as regent. When the nationalist group approached the 25-year-old for help, John readily agreed.


Queen Leonor
Queen Leonor

Fortunately for John of Aviz, he had more support than Queen Leanor. John was known as an intelligent, wealthy, and respected man. On the other hand, Leonor had an infamous reputation, and many viewed her as a traitor for her support of Juan I. In December 1383, John’s nationalist group invaded the royal palace, beginning the Portuguese Interregnum. In response, the queen fled the Portuguese capital of Lisbon for Castile. Upon Leonor’s departure, John became the de-facto ruler of Portugal.

In May 1384, the Castilian army invaded Portugal and attacked Lisbon. Initially expecting little resistance, the surprised Castilian army faced stubborn soldiers and Lisbon’s fortified walls. During the siege, John proved his worth as a military commander. After plague broke out, the Castilian army retreated. As a show of gratitude, the Portuguese named John defender of the realm.

By April 1385, John’s prestige had significantly risen. After an assembly met at Coimbra, the master of Aviz was elected Portugal’s new king. As a result, John became King John I of Portugal on April 6. Upon his accession, the king established the royal dynasty of Aviz.

Castilian Invasion

Although the king had widespread support, his position wasn’t secure. In the north, many Portuguese nobles still wanted a Castilian ruler. Due to this sentiment, the Castilians began preparing a massive invasion force. Aware of the upcoming invasion, John I contacted England for support.

With the help of his skilled constable Nuno Alvares Peira, John I began capturing pro-Castilian cities in the north. By July, the Castilian army invaded. Juan I of Castile had the support of King Charles VI of France. His army numbered 20,000 soldiers, dwarfing his rival’s 7,000. Upon Nuno’s advice, the Portuguese king decided to engage the Castilian army directly.

The Battle of Aljubarrota

John I of Castile
Juan I of Castile

At Aljubarrota, the Portuguese army created a defensive formation on a ridge. On August 14, 1385, an exhausted Castilian army arrived. Although Juan I attempted to avoid a direct assault, his unruly soldiers didn’t listen. As a result, Portuguese crossbowmen massacred the Castilian soldiers.

After the crossbowmen decimated the Castilian army, Juan I fled with his remaining troops. Upon Castile’s defeat, Portugal finally secured its independence. Furthermore, John I of Portugal’s victory secured his throne and increased Portugal’s prestige in Europe. This would also be the last time Castile made a serious attempt at subjugating Portugal.

Building a Dynasty

In May 1386, John I and Richard II of England signed the Treaty of Windsor. The treaty guaranteed mutual military support and created a trade partnership. It served as the beginning of an Anglo-Portuguese alliance. Shortly after, Richard sent aid to Portugal.

After his defeat at Aljubarrota, Juan I once again began planning to invade Portugal. To counter the Castilian king, John I of Portugal offered to support John of Gaunt’s claim to the Castilian throne. As part of their agreement, the king agreed to marry Gaunt’s daughter, Phillipa. Consequently, she provided John with five sons and a daughter. As queen, Phillipa strongly encouraged the Anglo-Portuguese alliance, which further strengthen both countries’ bonds.

John I and John of Gaunt’s military expedition ultimately proved fruitless. Having failed to make any significant progress, Gaunt accepted a substantial payment from Castile and departed. By October 1390, Juan I died, and his young son, Henry III, succeeded him. With the Castilian threat diminished, the Portuguese king could focus his attention elsewhere.


During the first two decades of the 1400s, John I began expanding Portuguese influence into Africa. On April 24, 1415, Ceuta, a port on the Moroccan coastline, was captured. The king built on this success by settling the Azores and Madeira Islands in the North Atlantic Ocean. John’s third son, Henry, also took a keen interest in overseas discoveries and sponsored many expeditions. Due to this, Portugal’s influence grew as its empire expanded.


Besides Prince Henry, John I had other children involved in royal affairs. The king’s heir, Edward, served his father as an able administrator. The king’s daughter, Isabel, married the Duke of Burgundy, which allowed the king to further his interest in northwestern Europe. Even the king’s illegitimate son assisted his father. Per John’s wishes, Afonso married Nuno Alvares’ daughter. Eventually, this marriage culminated in the house of Braganca, which would rule Portugal in 1640.


On August 14, 1433, the old king died from natural causes at the age of 76. Throughout his long life, John I had risen from the master of Aviz to Portugal’s ruler. After his death, John left a secure throne and prosperous kingdom to his son, Edward. The king’s children, known as the illustrious generation, would continue to build upon their father’s work. Through their efforts, Portugal expanded its empire and became more prestigious in Europe.


Disney, A. R. (2009). A History of Portugal and the Portuguese Empire: Volume One. New York, NY: Cambridge University Press.

Livermore, H. V. (2019, August 10). John I. Retrieved March 23, 2020, from


Andy Tree

I'm a European history enthusiast who seeks to share his passion with others. I hope to inform and inspire readers with my posts!


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