| Conflict: | Battle of Atoleiros |
| Partof: | 1383–1385 Crisis |
| Date: | 6 April 1384 |
| Place: | Atoleiros, Alentejo, Portugal |
| Result: | Portuguese victory[1] |
| Combatant1: | Kingdom of Portugal |
| Combatant2: | Crown of Castile |
| Commander1: | Nuno Álvares Pereira |
| Commander2: | Fernando Sanchez de Tovar Pedro Álvares Pereira Pero Gonzalez de Sevilla Martim Anes de Barundo |
| Strength1: | 1,400 men:[2] [3] |
| Strength2: | 5,000 men[7]
|
| Casualties1: | No casualties[9] [10] |
| Casualties2: | Heavy[11] |
The Battle of Atoleiros (pronounced as /pt/) 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 Fronteira[12] 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 Lisbon with 1,000 infantry, adding to the strength of his forces on his way to Atoleiros. The Castilian army consisted of some 5,000 men, mostly cavalry, which was besieging the village of Fronteira. As Pereira approached, the Castilians sent an emissary to him, attempting to persuade him to retire. He refused, and the Castilians advanced to meet him, lifting the siege. The Portuguese formed a defensive square. In the short battle that followed, the Castilian cavalry was unable to break the Portuguese formation, suffering heavy losses. The Portuguese suffered none, and the Castilians withdrew.
The battle of Atoleiros represents the first effective use of “square tactics” on the battleground. This tactic, in which groups of infantry armed with both missile and hand-to-hand weapons defended themselves from all directions, was so successful that it was still in use over 500 years later during the Napoleonic Wars against mass French cavalry attacks, and during the Zulu War against huge masses of predominantly spear-armed infantry. It was especially effective when the infantry had to fight against strong cavalry.