But Zumalacárregui, who was noted for his grave and silent disposition and his strong religious principles, disliked the disorderly life of the guerrillas, and when regular forces were organized in the north he entered the 1st battalion of Guipúzcoa as an officer. During the remainder of the war he served in the regular army. In 1812 he was sent with dispatches to the Regency at Cádiz, and received his commission as captain. In that rank he was present at the battle of San Marcial foist of August 1813. After the restoration of Ferdinand VII he continued in the army, and is said to have made a careful study of the theory of war.
In 1823 he returned as an officer in one of the royalist regiments which had been organized on French soil by the consent of the government. He was now known as a thoroughly trustworthy servant of the royalty, but he was too proud to be a courtier. For some years he was employed in bringing regiments which the government distrusted to order. He became lieutenant-colonel in 1825 and colonel in 1829. In 1832 he was named military governor of Ferrol. Before Ferdinand VII died in 1833, Zumalacárregui was marked out as a natural supporter of the absolutist party which favoured the king's brother, Carlos.
When the Carlist uprising began on the death of Ferdinand VII, he is said to have held back because he knew that the first leaders would be politicians and talkers. He did not take the field till the Carlist cause appeared to be at a very low ebb, and until he had received a commission from Don Carlos as commander-in-chief in Navarre.
The whole force under his orders when he escaped from Pamplona on the night of 29 October 1833, and took the command next day in the Arakil Valley, was a few hundred ill-armed and dispirited guerrilleros. In a few months Zumalacárregui had organized the Carlist forces into a regular army. The difficulty he found in obtaining supplies was very great, for the coast towns and notably Bilbao support the Cristino cause. It was mainly by captures from the government troops that he equipped his forces. He gradually obtained full possession of Navarre and the Basque Country, outside of the fortresses, which he had not the means to besiege. He organized the forces known as aduaneros and the Guías de Navarra. His chief bodyguard, and later biographer, was Charles Frederick Henningsen.
Whether as a guerrilla leader, or as a general conducting regular war in the mountains, he proved unconquerable. He won the battles of Alsasua, Alegría de Álava, and Venta de Echavarri, for example, by employing guerrilla tactics.
By July 1834 he had made it safe for Don Carlos to join his headquarters. The pretender was, however, a narrow-minded, bigoted man, who regarded Zumalacárregui with suspicion, and was afraid of his immense personal influence with the soldiers. Zumalacárregui had therefore to drag behind him the whole weight of the distrust and intrigues of the court. Yet by the beginning of June 1835 he had made the Carlist cause triumphant to the north of the Ebro, and had formed an army of more than 30,000 men, of much better quality than the constitutional forces. He won the battle of Artaza (April 20-22, 1835).
If Zumalacárregui had been allowed to follow his own plans, which were to concentrate his forces and march on Madrid, he might well have put Don Carlos in possession of the capital. But the court was eager to obtain command of a seaport, and Zumalacárregui was ordered to besiege Bilbao. He obeyed reluctantly, and on the 14th of June 1835 was wounded by a musket bullet in the calf of the leg near the Basilica of Begoña. The wound was trifling and would probably have been cured with ease if he had been allowed to employ an English doctor whom he trusted. But Don Carlos insisted on sending his own physicians, and in their hands the general died on 24 June 1835 not without suspicion of poison.
Zumalacárregui was a fine type of the old royalist and religious principles of his people. The ferocity with which he conducted the war forced the government generals to retaliate, as they were refused quarter. Zumalacárregui, however, had signed the Lord Eliot Convention, which aimed to end the indiscriminate executions by firing squad of prisoners of both sides.