The Thirty Years' War (1618-1648) was primarily caused by religious tensions between Protestant and Catholic states within the Holy Roman Empire. The conflict began when Protestant nobles revolted against the Catholic Habsburg monarchy, which was seen as imposing Catholicism on their territories. The war escalated, drawing in various European powers and leading to a complex political landscape. While religion was a significant factor, other motivations, including territorial disputes and the balance of power, also played crucial roles. Ultimately, the war devastated much of Central Europe, resulting in significant loss of life and shifts in political boundaries, culminating in the Peace of Westphalia.