After seven years as a soldier, Sierra was promoted to corporal in 2022. He passed the sergeant examination that year and graduated as a sergeant in 2025 before taking charge of a platoon with the VCZ Castor.
Military life was familiar to Sierra through his family, but he chose the Army for work beyond an office routine. During an interview at Barcelona's Bruch barracks, he described serving as a platoon leader in charge of the VCZ Castor, a combat-engineering vehicle described at the time as one of the Army's newest acquisitions. At the time of the interview, he was assigned to Batallón de Zapadores I of Brigada Aragón in Zaragoza.
His work has remained within Ingenieros, though his tactical role has changed. Before his current assignment, Sierra served with Regimiento de Pontoneros y Especialidades de Ingenieros número 12 in Zaragoza, where he worked as a diver. He described the branch as having a various roles.
The interview took place during a public opening of the Bruch barracks. That setting differs from the parliamentary process covered in Comín's oath-taking.
Sierra spent seven years as a soldier before being promoted to cabo in 2022. That year, he passed the competitive examination for sergeant. He became a sergeant in 2025, three years after passing the exam.
He requested a posting in Zaragoza, his hometown, and said he was pleased to receive it.
