1633 (Ring of Fire) by Eric Flint

The new government in central Europe, called the Confederated Principalities of Europe, was formed by an alliance between Gustavus Adolphus, King of Sweden, and the West Virginians led by Mike Stearns who were transplanted into 17th-century Germany by a mysterious cosmic accident. The new regime is shaky. Outside its borders, the Thirty Years War continues to rage. Within, it is beset by financial crisis as well as the political and social tensions between the democratic ideals of the 20th-century Americans and the aristocracy which continues to rule the roost in the CPE as everywhere in Europe.