1383 – 85 Crisis The 1383 - 1385 crisis or " Portuguese Interregnum " was a time of civil war in Portuguese history when no crowned king reigned . It began when King Ferdinand I of Portugal died without male heir s , and ended when King John I was crowned in 1385 , after the Battle of Aljubarrota . Portuguese interpret this era as their earliest national resistance movement countering Castilian intervention ; Robert Durand considers it the " great revealer of national consciousness " . Burgesses and nobles worked together to establish the Aviz dynasty ( a branch of the Portuguese House of Burgundy ) securely on an independent throne , unlike the lengthy civil wars in France known as the Hundred Years ' War , and England as the War of the Roses , where aristocratic factions fought powerfully against a centralised monarchy . Prelude . In 1383 , King Ferdinand I of Portugal was dying . From his marriage to Leonor Telles de Menezes only a girl , Princess Beatrice of Portugal , survived . Her marriage was the major political issue of the day , since it would determine the future of the kingdom . Several political factions lobbied for possible husbands , which included English and French princes . Finally , the king settled for his wife ' s first choice , King John I of Castile . Ferdinand had waged three wars against Castile during his reign , and the marriage , celebrated in May 1383 , was intended to put an end to hostilities by a union of the two crowns ; however , this was not a widely accepted solution . This dynastic union meant that Portugal would lose independence to Castile ; many nobles were fiercely opposed to this possibility , but they were not united under a common pretender to the crown . The two candidates , both illegitimate half - brothers of Ferdinand , were : On October 22 , King Ferdinand died . According to the marriage contract , Dowager Queen Leonor assumed regency in the name of her daughter Beatrice and son - in - law , John I of Castile . Since diplomatic opposition was no longer possible , the party for independence took more drastic measures , starting the 1383 – 1385 crisis . 1383 . The regent ' s privy council made the error of excluding any representation of the merchants of Lisbon . The first move was taken by the faction of John ( João ) of Aviz in December 1383 . João Fernandes Andeiro , Count of Ourém , called Conde Andeiro , the detested lover of the dowager queen , was murdered by a group of conspirators led by João of Aviz . Following this act , John , acclaimed " rector and defender of the realm " by the great Lisbon merchants , was now the leader of the opposition to the pretensions of John I of Castile , who tried to be recognized as monarch iure uxoris , against the Treaty of Salvaterra . 1384 . The armed resistance met the Castilian army on April 6 , 1384 , in the Battle of Atoleiros . General Nuno Álvares Pereira won the battle for the Aviz party , but victory was not decisive . John I of Castile then retreated to Lisbon in May and besieged the capital , with an auxiliary fleet blocking the city ' s port in the river Tagus , in a severe drawback to the independence cause . Without the capital and its riches and commerce , little could be done to free the country from the Castilian king . On his side , John I of Castile needed Lisbon , not only for financial reasons , but also for political ones — neither he nor Beatrice had been crowned as monarchs of Portugal , and without a coronation in the capital he was only a designated king . Meanwhile , John of Aviz had surrendered the military command of the resistance to Nuno Álvares Pereira . The general continued to attack cities loyal to the Castilians and to harass the invading army . John of Aviz was now focused on diplomatic offensives . International politics played an important role in deciding Portuguese affairs . In 1384 , the Hundred Years ' War was at its peak , with English and French forces in a struggle for the crown of France . The conflict spilled beyond the French borders , and influenced , for instance , the Western Schism in a papacy only recently moved to Avignon from Rome . Castile was a traditional ally of France , so , looking for assistance in England was the natural option for John of Aviz . In May , with Lisbon under siege , an embassy was sent to Richard II of England to make a case for Portuguese independence . In 1384 , Richard was seventeen years old , but the power lay with his uncle John of Gaunt , 1st Duke of Lancaster , and regent of England . Despite initial reluctance to concede men , John of Gaunt finally agreed to levy troops to reinforce the Portuguese army . Lisbon was struggling with famine and feared defeat by the Castilian siege . Blocked by land and by the river , the city had no hope of relief by the Aviz army , which was too small to risk an intervention and was occupied subduing other cities . An attempt was made by a Portuguese fleet to relieve the Castilian blockade . On July 18 a group of ships led by captain Rui Pereira managed to break the blockade and deliver precious supplies of food to Lisbon . The cost was high , since three of four boats were seized and Rui Pereira himself died in the naval combat . Despite this minor success , the siege held on ; the city of Almada on the south bank of the Tagus surrendered to Castile . But the siege was hard not only on the inhabitants of Lisbon : the army of Castile was also dealing with a shortage of food supplies , due to the harassment of Nuno Álvares Pereira , and the bubonic plague . It was the outbreak of an epidemic in his ranks that forced John I of Castile to raise the siege on September 3 and retreat to Castile . Weeks later , the Castilian fleet also abandoned the Tagus , and Lisbon avoided conquest . 1385 . In late 1384 and the early months of 1385 , Nuno Álvares Pereira and John of Aviz pursued the war , but they did not manage to subdue the majority of those Portuguese cities then in favour of the Castilian cause . Answering the call for help , English troops landed in Portugal on Easter Day . They were not a big contingent , around 600 men , but they were mainly veterans of the Hundred Years ' War battles and thereby well schooled in successful English military tactics . Among them were a small number of longbowmen who had already demonstrated their value against cavalry charges , as at Crécy . At the same time , John of Aviz organized a meeting in Coimbra of the " Cortes " , the assembly of the kingdom . There , on April 6 , he was proclaimed the tenth king of Portugal , a clear act of defiance against the Castilian pretensions . John I of Portugal nominated Nuno Álvares Pereira Constable of Portugal and went to subdue the resistance still surviving in the north . John I of Castile was not pleased . His first move was to send a punitive expedition , but the forces were heavily defeated in the Battle of Trancoso in May . From January , he began preparing his army to solve the problem definitively . The king himself led an enormous Castilian army that invaded Portugal in the second week of June through the central north , from Celorico da Beira to Coimbra and Leiria . An allied contingent of French heavy cavalry travelled with them . The power of numbers was on their side — about 32 , 000 men on the Castilian side versus 6 , 500 on the Portuguese . They immediately headed to the region of Lisbon and Santarém , the country ' s major cities . Meanwhile , the armies of John I of Portugal and Nuno Álvares Pereira joined together in the city of Tomar . After some debate , a decision was made : the Castilians could not be allowed to besiege Lisbon once again , since the city would undoubtedly fall , so the Portuguese would intercept the enemy in the vicinity of Leiria , near the village of Aljubarrota . On August 14 , the Castilian army , very slow due to its huge numbers , finally met the Portuguese and English troops . The ensuing fight , the Battle of Aljubarrota , was fought in the style of the Battles of Crécy and Poitiers . These tactics allowed a reduced infantry army to defeat cavalrymen with the use of longbowmen in the flanks and defensive structures ( like caltrop s ) in the front . The Castilian army was not only defeated , but annihilated . Their losses were so great that John I of Castile was prevented from attempting another invasion in the following years . Legacy . With this victory , John I was recognized as the undisputed king of Portugal , putting an end to the " interregnum " and anarchy of the 1383 – 1385 crisis . Recognition from Castile would not arrive until 1411 , after another Portuguese victory at the Battle of Valverde , with the signing of the Treaty of Ayllón . The English – Portuguese alliance would be renewed in 1386 with the Treaty of Windsor and the marriage of John I to Philippa of Lancaster , daughter of John of Gaunt . The treaty , still valid today , established a pact of mutual support between the countries . Indeed , Portugal would use it again against its neighbours in 1640 , to expel the Spanish Habsburg kings from the country , and again during the Peninsular War . The Anglo - Portuguese Alliance would also be used by Britain ( in succession from England ) in the Second World War ( allowing the Allies to establish bases on the Azores ) and during the 1982 Falklands War .