The First Battle of Olmedo was a pivotal conflict between the forces of John II of Castile and the Infantes of Aragon, occurring near the town of Olmedo. The battle emerged from the broader context of feudal tensions and political rivalries in the Iberian Peninsula. John II, who ascended to the Castilian throne during a turbulent period marked by noble disputes and regional power struggles, aimed to consolidate his rule against various factions, including the influential Aragonese nobility represented by the Infantes.

On the day of the battle, John II’s army, although outnumbered, was well-prepared. The Castilian troops employed effective tactics, including feigned retreats and ambush strategies, which played to their advantage. This unexpected approach ultimately led the Castilian forces to inflict significant casualties on the Aragonese, who were taken by surprise. The outcome of the battle reaffirmed John II's dynastic claims and authority over Castile, providing him with the opportunity to further strengthen his rule and diminish the prospects of rival claims to the Crown.

The victory was not merely a military achievement; it was also an essential step towards the unification of Spain, as it demonstrated the declining power of regional factions and the centralization of authority under the Castilian crown. The aftermath of the battle resulted in the consolidation of power for John II and set a precedent for future conflicts between noble factions in the region, thereby reshaping the political landscape of late medieval Spain.