Swedish king Gustavus Adolphus, has with the aid of the time-displaced citizens of Grantville, West Virginia tipped the balance in the Thirty Years' War and become emperor of much of Germany, now reorganized as the United States of Europe. Having at least temporarily knocked Austria and France, the main enemies of the new state, out of the war, he is free to turn his attention to the rebellious states of Brandenburg and Saxony and pursue his dream of conquering Poland. The former are duly reconquered and the latter invaded.