When the French invasion of Spain took place in 1808 he enlisted at Zaragoza. He served in the first siege, at the battle of Tudela, and during the second siege 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 Caspar de Jauregui, known as "The Shepherd" (El Pastor), one of the minor guerrillero leaders.
But Zumalacarregui, 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 Guipuzcoa as an officer. During the remainder of the war he served in the regular army. In 1812 he was sent with despatches to the Regency at Cadiz, 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. Zumalacarregui had no sympathy with the liberal principles which were spreading in Spain, and became noted as what was called a Servil or strong Royalist. He attracted no attention at headquarters, and was still a captain when the Revolution of 1820 broke out. His brother officers, whose leanings were liberal, denounced him to the revolutionary government, and asked that he might be removed. The recommendation was not acted on, but Zumalacárregui knew of it, and laid up the offence in his mind. Finding that he was suspected (probably with truth) of an intention to bring the soldiers over to the royalist side, he escaped to France.
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 despotic 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, Don Carlos.
The proclamation of the king's daughter Isabella as heiress was almost the occasion of an armed conflict between him and the naval authorities at Ferrol, who were partisans of the constitutional cause. He was put on half pay by the new authorities and ordered to live under police observation at Pamplona.
When the Carlist rising 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 the 29th of October 1833, and took the command next day in the Val de Araquil, was a few hundred ill-armed and dispirited guerrilleros. In a few months Zumalacarregui 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 ? were constitutional in politics. It was mainly by captures from the government troops that he equipped his forces. He gradually obtained full possession of Navarre and the Basque provinces, outside of the fortresses, which he had not the means to besiege.
Whether as a guerrillero leader, or as a general conducting regular war in the mountains, he proved unconquerable. 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.
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. 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.
Zumalacarregui was a fine type of the old royalist and religious principles of his people. The ferocity with which he conducted the war was forced on him by the government generals, who refused quarter.
An engaging account of Zumalacárregui will be found in "The Most Striking Events of a Twelvemonth Campaign with Zumalacarregui in Navarre and the Basque Provinces", by C. F. Henningsen (London, 1836). A chap-book called "Vida política y militar de Don Tomás Zumalacárregui", which gives the facts of his life with fair accuracy, is still very popular in Spain.
Some modern Basque nationalists believe that Zumalacárregui (or Zumalakarregi) was a precursor of their movement and that his opinions were more of Basque independence than Don Carlos's pretensions.