Ferdinand I of Portugal

Ferdinand I of Portugal
October 31, 1345 – October 22, 1383

Ferdinand I of Portugal was born in Lisbon, Portugal, on October 31, 1345. Born during the reign of his grandfather, Afonso IV, Ferdinand was the son of Prince Peter and Princess Costanza Manuel of Castile. In 1349, Costanza died, beginning a period of instability in Ferdinand’s life. Although his mother had given his father four children, Peter did not love her. Instead, the prince had been having an affair with a noblewoman named Ines de Castro since she arrived with Costanza in 1340.

Now a widow, Peter openly lived with Ines and their four illegitimate children. Due to his love for Ines, the prince preferred his children with his mistress over those with Costanza. To add more turmoil to Ferdinand’s life, Peter came into conflict with the king over his affair. Enraged at his son’s refusal to remarry anyone but Ines, Afonso IV allegedly ordered her death. In January 1355, assassins beheaded Ines in front of her children.

In response to Ines’ murder, Peter attempted to overthrow his father. However, the revolt would be short-lived as the two men reconciled. On May 28, 1357, Afonso IV died, and Peter I succeeded to the throne. Over the next ten years, Peter reigned over Portugal, avenging Ines’ death by hunting down and executing her assassins. Upon the king’s death on January 18, 1367, Ferdinand I succeeded to the throne.

King of Portugal

Unlike his father, Ferdinand I of Portugal was ill-suited to be king. He was known to be selfish, impulsive, and lacked a political mind. All three traits would influence his disastrous foreign policy towards Castile. Two years into Ferdinand’s reign, the Castilian king Peter was assassinated. Since Peter had no male heir, a succession crisis began in Castile.

As a descendent of King Sancho IV of Castile, Ferdinand I impulsively decided to press his claim to the throne. Breaking Portugal’s neutrality with Castile, the king began the first of three military campaigns. In 1369, Ferdinand invaded Castile and captured Galicia with little resistance. With the support of the French, Peter’s successor, Henry II, led a counter-invasion of Portugal. Panicked by Castile’s aggression, Ferdinand quickly agreed to a peace treaty in 1371 and renounced his claim to the throne.

Henry II of Castile
Henry II of Castile

Returning home with his reputation damaged, Ferdinand I had to deal with internal problems. His inner circle contained many foreign members, angering the Portuguese nobility. The king’s favoritism towards these members only aroused more hostility from the nobles. To make matters worse, Ferdinand made the rash decision to marry the sister of one of his favorites, Leonor Teles. As part of his peace treaty with Castile, the king had agreed to marry Henry II’s daughter, Leonor. By breaking the treaty’s terms, Castile resumed the war with Portugal.

Renewed Hostilities

In 1372, Portugal established a stronger relationship with England by allying with John of Gaunt. As King Edward III’s younger son, John had an interest in gaining his own throne. Since he was married to Constance of Castile, John wanted to seize the Castilian throne on her behalf. Agreeing to support John’s claim, Ferdinand I devised a two-prong invasion plan with him. John would invade from the north, while Ferdinand would invade from the west.

Despite the increased support, the invasion plan failed. John of Gaunt was forced to postpone his invasion, leaving Ferdinand I to face Henry II alone. In 1372, the Castilian king invaded Portugal, and his army quickly surrounded the capital. In early 1373, Castilian soldiers looted Lisbon’s outer suburbs and besieged its walls. Unprepared for the invasion, Ferdinand once again sued for peace. As part of the new treaty, the king renounced John, gave up various castles, and allowed Castilian troops to occupy Lisbon.

The terrible decisions of Ferdinand I of Portugal resulted in his kingdom experiencing famine, plague, and widespread destruction. As a result of the chaos, crime increased, provoking protests amongst the Portuguese. In response, Ferdinand promoted construction projects to rebuild his kingdom’s defenses and supported economic efforts to improve the economy. Surprisingly, the king’s actions produced positive results. However, Ferdinand’s domestic success would be overshadowed by another looming war with Castile.

  Final War With Castile

By the late 1370s, Portugal and England formalized their alliance through a treaty. Since England was engaged in the Hundred Years’ War with France, England needed the support of Portugal’s navy. Supporting the same pope during the Western Schism further cemented the two kingdom’s relationship. After Henry II died in May 1379, the king was succeeded by his son, John I. Seeing an opportunity to overthrow the new king, John of Gaunt and Ferdinand once again planned a joint invasion of Castile.

John I of Castile
John I of Castile

When English troops arrived in Lisbon in July 1381, the army demonstrated a lack of discipline and poor leadership. As the English alienated the Portuguese with their behavior, Castile invaded Portugal. Although John I captured some border towns and castles, his invasion made little progress. Despite having England’s support, Ferdinand I experienced little success in his campaign. Unable to sustain a long-term war, Ferdinand agreed to make a final peace with John in August 1382.

 Final Year

On April 30, 1383, Ferdinand I’s only legitimate child, Beatrice, married John I. The king hoped the marriage would unite Portugal and Castile under one dynasty, ensuring a lasting peace. Despite his good intentions, the Portuguese nobility feared Portugal would lose its independence. Ferdinand failed to gain their support as a result. Since Ferdinand was the last legitimate male of the Burgundian dynasty, a succession crisis brewed as the king lay dying. On October 22, Ferdinand died, ending the dynasty begun by Afonso I in 1139.


Ferdinand I of Portugal experienced a life full of turmoil. As a child, his father’s scandalous affair caused conflict within his family. As king, Ferdinand’s poor decisions brought destruction and chaos to Portugal. Although his policies experienced some domestic success, Ferdinand’s military failures in Castile stained his reign. By the time he died, Ferdinand had left behind a kingdom in civil unrest, beginning the Portuguese Interregnum.


Disney, A. R. (2009). A History of Portugal and the Portuguese Empire: From Beginnings to 1807 (Vol. 1). New York City, NY: Cambridge University Press.

Encyclopædia Britannica, inc. (n.d.). Ferdinand I. Encyclopædia Britannica. Retrieved January 13, 2023, from https://www.britannica.com/biography/Ferdinand-I-king-of-Portugal


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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