The morale of the Carlists was very high, and they decided to fight the Liberal troops in a formal battle (rather than with guerrilla tactics).
Tomás de Zumalacárregui concentrated his forces at the bottom of the valley of La Berrueza between Mendaza and Asarta.
Zumalacárregui attempted to follow a plan of battle similar to that enacted by Hannibal at Cannae: he would allow enemy forces to drive themselves into a large arc — whereupon the Carlist infantry, positioned on the flanks in the forests of Holm oaks on the mountain of Dos Hermanas, would encircle the main body of Liberal infantry and destroy it.
However, the Liberal leader of the vanguard, Marcelino Oráa, himself a Navarrese and familiar with the terrain, recognized the potential trap and instead marched towards Mendaza rather than through the valley itself.
The Carlists, surprised by this maneuver and inexperienced on the field of battle, were thrown back and retreated, taking refuge in the mountains.