Berlin is the capital and the largest city of Germany as well as one of its 16 constituent states. With a population of approximately 3.7 million, Berlin is the second most populous city proper in the European Union and the seventh most populous urban area in the European Union. Located in northeastern Germany on the banks of the rivers Spree and Havel, it is the centre of the Berlin-Brandenburg Metropolitan Region, which has roughly 6 million residents from more than 180 nations. Due to its location in the European Plain, Berlin is influenced by a temperate seasonal climate. Around one-third of the city's area is composed of forests, parks, gardens, rivers, canals and lakes.First documented in the 13th century and situated at the crossing of two important historic trade routes, Berlin became the capital of the Margraviate of Brandenburg, the Kingdom of Prussia (1701–1918), the German Empire (1871–1918), the Weimar Republic (1919–1933) and the Third Reich (1933–1945). Berlin in the 1920s was the third largest municipality in the world. After World War II and its subsequent occupation by the victorious countries, the city was divided; East Berlin was declared capital of East Germany, while West Berlin became a de facto West German exclave, surrounded by the Berlin Wall (1961–1989) and East German territory. Following German reunification in 1990, Berlin once again became the capital of all of Germany.