Germany was a war criminal and defeated nation in the World War II of 1945, but as the Berlin Wall fell on October 9, 1989, it became an officially unified nation on October 3, 1990. It, as a war criminal, was forced to be divided into West Germany and East Germany and managed by four powerful countries of the USA, England, France and Russia. Until the reunification, West Germany introduced liberal democracy system and market economy from the western world and became one of advanced countries while as East Germany introduced socialism and planned economy from Russia, it was remarkably behind in national power in comparison with that of West Germany. As for the reunification of Germany, West Germany assimilated East Germany. Domestically, East and West Germany continued to proceed personal and material exchanges in public and private levels even under the cold war system and division condition. International factors were: First, Russia and socialist nations collapsed and the cold war system in the East and the West terminated; Second, the USA started a multiple system containing England, France, Russia, East and West Germany which were active in Germany reunification and tangible results were achieved; Third, NATO and EU had the same political, military, security and economic interests and strategic goals. There were social, economic and psychological conflicts between West and East Germany after the reunification, but as time lapsed, the internal conflicts were gradually mitigated. South and North Korea, and China and Taiwan should carefully consider such conflicts inherent in Germany's reunification for the process of reunification.