William Walker believed in manifest destiny with a fervor that went beyond rhetoric. His plan, which he titled "Five or None," aimed at nothing less than the conquest of all five Central American republics -- Guatemala, El Salvador, Honduras, Nicaragua, and Costa Rica -- to forge a new English-speaking domain where slavery would be legal. Most Latin American nations had already abolished slavery, which made Walker's vision both retrograde and dangerous. He established himself in Nicaragua in 1855 by exploiting a civil war between Liberals and Conservatives, arriving with a small band of American mercenaries under the guise of offering military aid to the Liberals. Within months, he had maneuvered himself into the presidency. From Managua, he began eyeing the nations to the south, and Costa Rica understood what was coming.

Costa Rican President Juan Rafael Mora Porras recognized the threat before his neighbors did. Urged on by British diplomatic interests -- who had their own reasons for opposing American expansion in the region -- Mora declared war on Nicaragua on February 27, 1856, and issued a call to arms that resonated across the country. On March 4, the president himself led the Costa Rican army out of San Jose, marching north toward the Nicaraguan border. They reached Liberia on March 12, where they joined the Moracia Battalion under the command of Jose Maria Canas. The combined force numbered roughly 9,000 troops -- an enormous mobilization for a small nation, and a measure of how seriously Costa Rica took the invasion threat. When Walker's commanders learned of the Costa Rican advance, they dispatched a battalion of about 70 men under Colonel Louis Schlessinger to intercept them.

Schlessinger's force was multinational: two companies of Frenchmen and two of Germans, all serving under Walker's banner. They entered Costa Rica on the road from Nicaragua to Liberia and reached Hacienda Santa Rosa late on the night of March 20, exhausted from a long march. The filibusters stationed themselves in the casona -- the hacienda's main house -- and the surrounding corrals, but they failed to post adequate sentries. At four o'clock that afternoon, the Costa Ricans arrived. Armed with rifles, sabres, and bayonets, they surrounded the hacienda and launched a sudden assault. The German company broke first, fleeing the field. The French under Captain Legaye tried to hold broken ground nearby but were quickly pushed back. Within five minutes the entire command was in disarray. In fourteen minutes the battle was over. Walker's troops lost 59 killed; the Costa Ricans lost 20. The remnants of Schlessinger's force retreated north in disorder.

The Battle of Santa Rosa was the opening engagement of the Filibuster War, but Walker's defeats did not end there. At the Second Battle of Rivas in April 1856, Costa Rican forces fought again -- this time suffering heavy casualties but holding the line. Walker was eventually driven from Nicaragua by a coalition of Central American armies. He fled north, attempted further invasions of Honduras, and was finally captured by the Honduran government, tried, and executed by firing squad in 1860. For Costa Rica, Santa Rosa became something larger than a military engagement. It became a symbol of national sovereignty and collective will -- a story of farmers and shopkeepers who marched north to defend their country against a foreign adventurer. The battlefield's centerpiece, the Casona de Santa Rosa, was damaged by arson in 2001 but has since been rebuilt and now serves as a museum within Santa Rosa National Park.