Hernán Cortés, after conquering the city of Tenochtitlan, capital of the Aztec empire, delegated the conquest of the territories southward to his lieutenant Pedro de Alvarado, who set out with 120 horsemen, 300 foot soldiers, and several hundred Cholula and Tlaxcala auxiliaries.

After subduing the highland Mayan city-states of present-day Guatemala through battle and co-optation, the Spanish sought to extend their dominion to the lower Atlantic region of the Nahuat-speaking people whom his translators called "Pipils," then organized as the powerful state of Cuzcatlán.

The Kaqchikel Mayans, who had long been rivals of Cuzcatlán for control over their wealthy cacao-producing region, joined forces with Alvarado's men and supported his campaign.

Receiving word of the approaching Spanish forces, the Pipil peasants who lived in the nearby villages of Mopicalco and Acatepeque fled.

According to records, a battle ensued between the opposing armies, with the Pipil wearing cotton armor (of three fingers' thickness, according to Alvarado) and carrying long spears.