The concept of Germany as a distinct region can be traced to Julius Caesar, who referred to the unconquered area east of the Rhine as "Germania", which distinguished it from Gaul (France). In the Late Middle Ages, the regional dukes, princes and bishops gained power at the expense of the emperors, and Martin Luther led the Protestant Reformation against the Catholic Church after 1517. The northern German states became Protestant, while the south remained Catholic. These two parts of the Holy Roman Empire clashed in the Thirty Years' War (1618-1648). The separate German states that ensued were gathered back into a single German state by Otto von Bismarck, forming the German Empire in 1871. Germany's industrial power grew over the decades leading to World War 1, which was again ruinous for the country. The rise of Nazism and Adolph Hitler in the 1930s resulted a second ruinous war that cost the lives of 9 million Germans, 6 million Jews, and tens of millions of others. Since World War 2 Germany has prospered and become a leading member of the European Union.