From 1512 to 1515, Ulrich Zwingli was a chaplain in the Papal forces of Pope Julius II during the wars against King Louis XII of France. This was an important phase in the life of Zwingli as it opened his eyes to the cruelties of war and the intricacies of political and religious alliances in early 16th-century Europe. As a chaplain, Zwingli’s primary responsibility was to provide spiritual care and moral support to the soldiers. He conducted prayers, offered sacraments, and delivered sermons to uplift the troops and bolster their resolve amid the chaos of battle.

Zwingli’s position as a non-combatant allowed him to witness the battlefield’s brutal realities without participating directly in the violence. He saw firsthand the death, devastation, and misery that war visited not only on soldiers, but on civilian populations trapped in between. These experiences made a profound impression upon him, and they shaped both his views on war and his views of the church's proper role in earthly conflicts. Although Switzerland was known for its mercenary tradition, Zwingli began to develop strong anti-mercenary sentiments, believing that such practices degraded the moral character of his nation and drew the Swiss people into unnecessary bloodshed for political or financial gain.

Zwingli’s time with the Papal armies also exposed him to the political entanglements of the church. He observed how religious leaders often prioritized alliances and power over spiritual integrity. This disillusionment with the church’s political ambitions contributed to Zwingli’s growing desire for reform. His experiences during these years laid the groundwork for his later advocacy of separating church and state affairs and focusing the church’s mission on spiritual matters rather than military and political ventures. Ultimately, his chaplaincy experience helped to forge Zwingli’s identity as a reformer committed to challenging the corruption and worldliness he witnessed within the church hierarchy.