When the Peninsular War began with a French invasion of Spain in 1808 he enlisted in Zaragoza. He served in the 1808 First Siege of Zaragoza, at the Battle of Tudela, and during the 1809 Second Siege of Zaragoza until he was taken prisoner in a sortie. He succeeded in escaping and in reaching his family in Navarre. For a short time he served with Gaspar de Jáuregui, another Gipuzkoan known as "The Shepherd" (), one of the guerrilla leaders, who later became a general in the regular army against which Zumalacarregui fought.