After the unification of the German Empire, Breslau became Germany's sixth largest city. After World War I, the city became part of the Province of Lower Silesia during the Weimar Republic. It also became a center of Polish nationalism. In the 1930s, it was a center of support for Nazism. It was left untouched by World War II until January, 1945, when it became the target of a costly siege by the Soviet Red Army. The city was officially ceded to Poland after the war. The name "Wroclaw" was officially restored, and the German population was expelled or fled in the years after the war.