Philippa of Lancaster and the Anglo-Portuguese Alliance (1387–1430)

In 1387, Philippa of Lancaster, daughter of John of Gaunt, married King João I of Portugal, sealing the Anglo-Portuguese alliance, one of the longest-standing diplomatic alliances in history. Through this union, João and Philippa became the parents of a remarkable generation of princes, whom poet Luís de Camões would later call the "marvelous generation", responsible for leading Portugal into its golden age.

Philippa’s Influence: Education, Morality, and Commerce

Philippa brought to the Portuguese court the Anglo-Norman tradition of aristocratic education, ensuring her children received a rigorous intellectual and moral upbringing. She reformed the royal court, instilling strict standards of morality and discipline, shaping Portugal’s royal culture for generations.

Beyond courtly influence, Philippa also provided royal patronage for English commercial interests, fostering trade between Portugal and England. English merchants supplied cod and cloth, while Portuguese traders exported wine, cork, salt, and olive oil through English warehouses in Porto, strengthening economic ties between the two nations.

The "Marvelous Generation" and Their Achievements

Philippa’s sons were among the most accomplished figures in Portuguese history:

• Duarte (Edward I of Portugal) – A scholar-king, he authored moral treatises and ruled as King of Portugal from 1433 until his death in 1438.
• Pedro, Duke of Coimbra – A well-traveled intellectual, deeply interested in history and governance, who later served as regent of Portugal after Duarte’s death.
• Fernando, the Saint Prince – A crusader, he participated in the failed 1437 attack on Tangier, where he was captured and later died in captivity.
• Henrique (Prince Henry the Navigator) – Master of the Order of Avis, he became the driving force behind Portugal’s early voyages of discovery, laying the groundwork for the Age of Exploration.

Philippa’s Enduring Legacy

Philippa of Lancaster’s legacy extended beyond diplomacy; through her court reforms, patronage, and maternal influence, she shaped the cultural, political, and economic foundations of Portugal’s golden age. Her children’s military, intellectual, and exploratory achievements propelled Portugal to the forefront of European expansion and discovery in the 15th and 16th centuries.

The 1383–1385 Crisis: Portugal's Struggle for Independence

By the late 14th century, Europe was in a state of crisis, plagued by the Hundred Years' War, the Black Death, and widespread famine. In Portugal, the situation was no different—following the death of King Ferdinand I in October 1383, the kingdom faced a dynastic and national crisis that threatened its independence from Castile.

The Treaty of Salvaterra de Magos and Portuguese Discontent (1383)

• In April 1383, King Ferdinand I had signed the Treaty of Salvaterra de Magos with John I of Castile, ensuring that:
  • Princess Beatrice, Ferdinand’s only child, would marry John I of Castile.
  • The Portuguese Crown would pass to their descendants, effectively setting the stage for a dynastic union with Castile.
• The treaty excluded the Portuguese nobility and merchants from negotiations, enraging the population and fueling fears that Portugal would be absorbed into Castile.
• With no undisputed heir, Portugal entered an interregnum (1383–1385), with no recognized monarch.

The Assassination of Count Andeiro and the Rise of João of Aviz (December 1383)

• The first open act of rebellion occurred in December 1383, when João, Grand Master of the Order of Aviz(illegitimate son of Peter I of Portugal), led the assassination of Count João Fernandes Andeiro, a pro-Castilian noble.
• The merchants of Lisbon, outraged by Castilian influence, named João of Aviz "Rector and Defender of the Realm", making him the leader of the anti-Castilian resistance.

John I of Castile’s Invasion and the Portuguese Response (1384)

• John I of Castile refused to abandon his claim to the Portuguese throne and moved to secure power by forcing Queen Leonor Teles to abdicate.
• In April 1384, he launched a punitive expedition into Alentejo.
• His Castilian forces were met at the Battle of Atoleiros (April 1384) by a smaller Portuguese army led by Nuno Álvares Pereira, a brilliant general and loyal supporter of João of Aviz.
• Using English defensive tactics, Pereira’s well-positioned infantry repelled Castilian cavalry charges, reportedly without a single Portuguese casualty.

The Beginning of the Fight for Independence

• Though a tactical success, the Battle of Atoleiros did not end Castilian ambitions.
• John I of Castile prepared for a larger invasion, culminating in the siege of Lisbon (1384) and later, the Battle of Aljubarrota (1385), which would determine Portugal’s fate as an independent nation.

The 1383–1385 Crisis was more than just a succession dispute—it became a national struggle for Portugal’s sovereignty, ultimately leading to the rise of the House of Aviz and the consolidation of Portuguese independence.

John of Gaunt’s Claim to the Castilian Throne and His 1386 Campaign in Spain

John of Gaunt, Duke of Lancaster, had long pursued a claim to the throne of Castile and León through his marriage to Infanta Constance of Castile, daughter of Peter I of Castile ("Peter the Cruel").

John of Gaunt’s Castilian Kingship in Exile (1372–1386)

• Upon his marriage to Constance of Castile in 1371, John of Gaunt formally assumed the title of "King of Castile and León" on January 29, 1372.
• He insisted on being addressed as "my lord of Spain" and impaled his personal arms with those of Castile and León.
• From 1372 onward, he established a small Castilian court-in-exile, gathering refugee Castilian nobles and knights.
• He set up a Castilian chancery, which produced documents styled after those of Peter I, signed with the Spanish royal formula "Yo El Rey" (I, the King).
• Despite his ambitions, his attempts to invade Castile repeatedly failed due to financial constraints, conflicting English wars with France and Scotland, and lack of diplomatic support.

The 1386 Invasion: John of Gaunt’s Final Attempt to Take Castile

With Portugal and England forming a full alliance in 1386 under the Treaty of Windsor, John of Gaunt finally launched his long-delayed military campaign to seize Castile.

• July 9, 1386 – He sailed from England at the head of a huge Anglo-Portuguese fleet, carrying:
  • An army of about 5,000 English troops, including longbowmen and knights.
  • His "royal" household, reinforcing his claim as King of Castile.
  • His wife, Constance, and daughters, including Philippa of Lancaster, who would later marry John I of Portugal.
• En route to Iberia, his forces relieved the besieged English stronghold of Brest, expelling French troops.
• July 29, 1386 – He landed at Corunna in northern Spain, officially beginning his military campaign to claim the Castilian throne.

The Campaign and Its Failure

• John of Gaunt advanced into Galicia, but his campaign soon stalled due to:
  • Harsh conditions and lack of local support.
  • Disease and supply shortages that weakened his forces.
  • The superior Castilian-French alliance, which effectively countered his military efforts.
• By 1387, his campaign faltered, leading to negotiations with Castile.
• In 1388, under the Treaty of Bayonne, John of Gaunt abandoned his claim to the Castilian throne in exchange for a large financial settlement and the marriage of his daughter, Catherine of Lancaster, to Henry III of Castile, helping to unite the rival Castilian dynasties.

Aftermath and Legacy

• Though his invasion failed, John of Gaunt’s campaign:
  • Strengthened the Anglo-Portuguese alliance, reinforcing Portugal’s independence from Castile.
  • Ensured that his Lancastrian lineage remained tied to Iberian politics through marriage alliances.
  • Redirected England’s focus back to France, where the Hundred Years’ War continued.

John of Gaunt’s long-standing ambition to rule Castile ended in disappointment, but his marriage diplomacy would leave a lasting impact on Iberian and English royal politics for generations.

The Failure of John of Gaunt’s Castilian Campaign (1387) and His Treaty with Castile

By 1387, John of Gaunt’s long-anticipated invasion of Castile, launched in April, had collapsed into failure, plagued by disease, logistical issues, and ineffective military strategy.

The Struggles of the Invasion (April–June 1387)

• John’s army was already weakened—many of his troops had succumbed to sickness before the campaign even began.
• His English forces were far outnumbered by their Portuguese allies, reducing his ability to wage an effective offensive.
• The Castilians, led by John I of Castile, refused to offer pitched battle, instead using harassment tactics carried out by French mercenaries.
• The Galician-Anglo-Portuguese troops were forced to rely on foraging in the arid Spanish landscape, leading to severe food shortages.

The Campaign’s Collapse

• The campaign became bogged down in time-wasting sieges of fortified towns, failing to achieve any major victories.
• Hundreds of English troops, including John of Gaunt’s personal friends and retainers, died from disease and exhaustion.
• Desertions increased, and many English knights abandoned the campaign, negotiating safe passage through France to return home.
• By June 1387, the army had returned to Portugal, exhausted and demoralized.

The Treaty of Bayonne: John of Gaunt Renounces His Claim to Castile

• Shortly after the campaign’s failure, John of Gaunt entered into secret negotiations with John I of Castile.
• In the Treaty of Bayonne (1388), the following terms were agreed upon:
  • John of Gaunt and his wife, Constance of Castile, formally renounced their claim to the Castilian throne.
  • In return, Gaunt received a large annual payment from the Castilian crown.
  • Catherine of Lancaster, Gaunt’s daughter, was betrothed to Henry III of Castile, the son of John I of Castile, ensuring dynastic reconciliation between the rival Castilian factions.

Aftermath and Legacy

• The failed campaign marked the end of John of Gaunt’s ambitions in Spain, forcing him to return to England.
• His daughter Catherine’s marriage to Henry III of Castile helped unify the Trastámara dynasty, ensuring stability in Castile.
• The Anglo-Portuguese alliance remained strong, leading to the Treaty of Windsor (1386), which solidified the long-standing alliance between England and Portugal.

John of Gaunt’s Castilian adventure, once a grand ambition to claim a kingdom, ended in humiliating failure, but his diplomatic maneuvering secured peace with Castile and ensured that his family remained influential in Iberian politics for generations to come.

John of Gaunt’s Galician Campaign and the Treaty of Ponte do Mouro (1386–1387)

John of Gaunt’s long-planned invasion of Castile in 1386 caught King John I of Castile (John of Trastámara) off guard. Expecting Gaunt to land in Portugal, the Castilian king concentrated his forces along the Portuguese border, only to find that Gaunt had chosen to invade Galicia instead—a region that was both the most distant from Castilian control and disaffected with Trastámara rule.

The Occupation of Galicia (August–October 1386)

• From August to October 1386, John of Gaunt established a rudimentary court and chancery in Ourense, marking his attempt to govern Galicia as King of Castile and León.
• The Galician nobility and most of the region’s towns pledged their allegiance to him, though their support was conditional—they would only recognize his rule if the rest of Castile followed suit.
• Despite this initial success, Gaunt failed to provoke the decisive battle he had hoped for, as the Castilian forces avoided direct engagement.

Challenges and the Agreement with Portugal (November 1386)

• As the campaign dragged on, Gaunt’s army suffered from dwindling supplies, disease, and the inability to secure full Castilian support.
• Keeping his troops paid and together became increasingly difficult, forcing him to seek further assistance.
• In November 1386, he met with King John I of Portugal at Ponte do Mouro, on the southern bank of the Minho River, to negotiate a joint Anglo-Portuguese strategy against Castile.

The Treaty of Ponte do Mouro and the Marriage of Philippa of Lancaster

• Gaunt and John I of Portugal agreed to launch a joint Anglo-Portuguese invasion of central Castile in early 1387, hoping to force John of Castile into open battle.
• The alliance was sealed by the marriage of John of Gaunt’s eldest daughter, Philippa of Lancaster, to King John I of Portugal.
• This marriage cemented the Anglo-Portuguese alliance, which had been growing since the Treaty of Windsor (1386) and remains the longest-standing diplomatic alliance in history.

Aftermath and Legacy

• Despite this agreement, Gaunt’s campaign ultimately failed, as his troops continued to struggle with disease, desertions, and logistical challenges.
• By 1388, Gaunt negotiated the Treaty of Bayonne, renouncing his claim to Castile in exchange for financial compensation and the marriage of his daughter Catherine to Henry III of Castile.
• The marriage of Philippa of Lancaster and John I of Portugal in 1387 produced a lasting Anglo-Portuguese alliance, leading to closer military and economic ties between the two kingdoms.
• Philippa became Queen of Portugal and the mother of the "Illustrious Generation", including Henry the Navigator, who played a key role in Portugal’s Age of Exploration.

Thus, while John of Gaunt’s Castilian ambitions ended in failure, his diplomatic efforts strengthened Portugal’s independence and ensured that England and Portugal remained allies for centuries to come.

John of Gaunt’s Withdrawal from Castile and the Anglo-Portuguese Alliance (1388)

After his unsuccessful invasion of Castile, John of Gaunt, Duke of Lancaster, formally abandoned his claim to the Castilian throne in 1388, concluding peace at Bayonne in exchange for a substantial financial settlement from King John I of Castile. At the same time, his dynastic ambitions shifted to Portugal, as his daughter, Philippa of Lancaster, married King John I of Portugal, solidifying an Anglo-Portuguese alliance.

Additionally, the informal truce between Castile and Portugal (established in 1387) was extended, allowing time for an official peace treaty to be arranged between the Iberian kingdoms.

Key Developments Leading to the 1388 Peace Agreement

1. John of Gaunt’s Invasion of Castile (1386–1387) and Failure

   • Gaunt had asserted a claim to the Castilian throne through his marriage to Constance of Castile, daughter of the deposed King Peter I ("the Cruel").
   • His military campaign in Castile (1386–1387), launched in alliance with Portugal, failed to gain significant ground.
   • Facing logistical difficulties, lack of local support, and superior Castilian resistance, he withdrew from the campaign.

2. Marriage of Philippa of Lancaster and John I of Portugal (1387)

   • Seeking to secure a lasting alliance with Portugal, Gaunt arranged for his daughter Philippa of Lancaster to marry King John I of Portugal.
   • This marriage strengthened Anglo-Portuguese ties and helped Portugal resist Castilian influence.

3. The Peace of Bayonne (1388): Gaunt Renounces Castile

   • Gaunt formally relinquished his claim to Castile, recognizing John I of Castile as the legitimate ruler.
   • In exchange, he received a large financial settlement, bringing an end to his unsuccessful Iberian ambitions.

4. The Extension of the Truce Between Castile and Portugal (1387–1388)

   • With Gaunt out of the picture, Castile and Portugal extended their truce, preparing for a lasting peace agreement.

Impact and Legacy

• John of Gaunt’s withdrawal from Castile ended English intervention in Castilian affairs, redirecting Gaunt’s focus to England and France.
• The marriage between Philippa of Lancaster and John I of Portugal established the Anglo-Portuguese Alliance, which would endure for centuries.
• The truce between Castile and Portugal paved the way for a formal peace treaty, reducing conflicts in the Iberian Peninsula.

The 1388 Peace of Bayonne marked the definitive end of John of Gaunt’s claim to Castile, while the Anglo-Portuguese marriage created a diplomatic bond that would influence European politics for generations.