The Thirty Years' War was the ultimate clash of religious battles in Europe. It started when the Holy Roman Emperor forced his Bohemian subjects back to Roman Catholic. After not complying war broke out, mainly in the German area. By the end Spanish, French, Dutch, German, Swedish, Danish and Polish forces all had their fair share of fighting in it as well. This war was the most destructive conflict in Europe before the 20th century. For this, the Thirty Years' War deserves 4.3 for the Holy Roman Empire trying to regain religious power.