Germany is a country located in Central Europe, bordered by Poland, the Czech Republic, Austria, Switzerland, France, Luxembourg, Belgium, the Netherlands and Denmark. The country has mountainous terrain in the south, dominated by the Alps, with a plateau and forests in the center and the north. The Rhine, the Danube and the Elbe rivers, combined with Germany's central location in Europe and its access to the North Sea, allowed the country to become a leading exporter and one of the most prosperous economies in Europe. Between the late 10th and early 19th centuries, the territories of today's Germany were the heart of the Holy Roman Empire, a collection of small kingdoms, principalities, duchies and city-states. This led to the development of multiple seats of political and economic power that achieved unity in 1871. While Berlin is Germany's capital and most populous city, the country also has several economic and political centers, including Hamburg, Munich, Cologne and Frankfurt. Germany’s location in the heart of the North European Plain has led to constant conflicts with its neighbors like France and Russia. After World War II, Germany was divided between East Germany, in the Soviet orbit, and West Germany, integrated to the European Economic Community and NATO. The country achieved reunification in 1990. Germany’s main geographic challenge is preserving its territorial unity and maintaining a political balance between regions within the country. It also seeks to maintain a political alliance with France and a balance of power in Europe to preserve peace and keep markets open for trade. Berlin's efforts to keep the European Union closely integrated amid the current economic crisis are in line with this strategy.