[1] The bourgeoisie and the nobility worked together to establish the Aviz dynasty, a branch of the Portuguese House of Burgundy, securely on an independent throne.

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 but was not a widely-accepted solution.

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.

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.

The conflict spilled beyond the French borders, and influenced, for instance, the Western Schism in a papacy only recently moved to Avignon from Rome.

Despite initial reluctance to concede men, John of Gaunt finally agreed to levy troops to reinforce the Portuguese army.

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.

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.

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.

They were not a big contingent, around 600 men (of which about 100 would be present in Ajubarrota), but they were mainly veterans of the Hundred Years' War battles and thereby well schooled in successful English military tactics.

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.

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.

These tactics allowed a reduced infantry army to defeat cavalrymen with the use of longbowmen in the flanks and defensive structures (like caltrops) in the front.

[4] The Castilian forces refused to offer battle, after two months no significant town was taken and the allies, struck by disease and lack of supplies, met with an overwhelming failure.