Happened on December 22, 1856, once Costa Rica resumed the National Campaign, the Expeditionary Army was divided into two units: the first, made up of 700 men and under General José María Cañas, and the second called Vanguard Division and formed by 200 soldiers, it made it through the plains of San Carlos towards the San Juan river.
This second troop had the important task of taking over the Transit route, since this was the main route by which the filibuster army provided resources. Mastery of the Transit route, the most important strategic point in the entire conflict, assured victory for either side.
On December 21, they reached the Estero de Colpachí, where they spent the night. On December 22, at 12:30 p.m., the Costa Ricans attacked the filibusters from three flanks: a column of 30 men under the orders of Major Blanco took the right flank, while the rest would attack from the center and left, following the command of Lieutenant Colonel Fernandez and Captain Spencer.
In forty minutes, by means of a bayonet attack, they managed to seize La Trinidad with very few casualties. While the filibusters lost 60 men and 6 prisoners were captured, the Costa Ricans had 9 dead and 10 wounded.
During this battle, a farmer from the town of Barva stood out, named Nicolás Aguilar Murillo, who would have been the first to jump out of his trench and seize an enemy cannon, and after killing the sentry, he engaged in hand-to-hand combat with a filibuster with a reputation for being a fierce man, surnamed Thompson, whom he defeated, leaving him wounded.
The La Trinidad combat was a short but vital skirmish in the objective of the Costa Rican Army to break through to control the Transit route. From the strategic point of view, it is considered a fundamental battle of the National Campaign, since it allowed the subsequent taking of the filibuster steamers that navigated the San Juan River and that provided troops and supplies to William Walker.