The Thirty Years' War, 1618-1648, was a series of conflicts that became the last great struggle of religious wars in Europe. It was fought almost exclusively on German soil...but before the war ended, it involved most of the nations of Europe. The underlying cause of the war was the deep-seated hostility between the German Protestants and German Catholics - with the Jesuits and Cardinal Richelieu, who was the real ruler of France, fanning the fires to accomplish their ends.