The Battle of Atoleiros took place on 6 April 1384, between a Portuguese force and a punitive expedition from Castile sent by John I. The battle took place near the population centre of the same name in Alentejo. It was the first major battle of the 1383–1385 Crisis.
Nuno Álvares Pereira had been chosen to protect the frontier in this area, amid fear that a Castilian force could enter Portugal here. He left from Lisbon, adding to the strength of his forces on his way to Atoleiros. As he approached the Castilian army, the Castilians sent an emissary in to the Portuguese army, attempting to dissuade it. The Portuguese refused, and when the Castilians advanced to meet them, breaking the ongoing siege to the village of Fronteira. The Portuguese formed a defensive square and in the short battle that followed, the Castilian cavalry was unable to break the Portuguese formation, and withdrew.
The battle of Atoleiros represents the first time where we see the effective use of the “square tactics” on the battleground. This famous tactic, widely used by British troops, consisted of defense techniques used by infantry especially when they had to fight against a strong heavy cavalry enemy side while in numerical disadvantage.
In this precise battle the Castilians suffered heavy losses and the Portuguese neither registered any deaths nor wounded people among their ranks