The United States, Great Britain, France, and the Soviet Union (The Allied Nations)fought together against Germany, Italy, Japan (the Axis Powers) and during World War II. Both factions included many smaller countries, as well.
When the war was over, much of Europe was in ruins. Germany was a broken country, with it's infrastructure destroyed and its people unable to help themselves. The Allied countries, with the exception of the Soviet Union, wanted to rebuilt Germany so that she could become self-sufficient again, under the Marshall Plan. The Soviet Union wanted to annex Germany as an Eastern Bloc country, so the she would be under Soviet Rule.
As a result of this disagreement, Germany was divided into two countries, East Germany and West Germany. East Germany became a socialist Eastern Bloc country under the Soviet Union, and it's official name was the German Democratic Republic or GDR. West Germany's official name was the Federal Republic of Germany, or FRG. American, British and French troops remained in West Germany throughout the rebuilding, and in 1955, the country was declared "fully sovereign," or operating on its own, with it's own government. West Germany joined the North Atlantic Treaty Organization (NATO), and the Allied troops maintain bases there until this day, under NATO.
Throught the 1950s, many people attempted to defect from East to West Germany. The easiest place to do this at the time was in Berlin. Although the entire city was in East Germany, half of the city was controlled by the US, Great Britain and France.
In 1961, the Soviets built the Berlin Wall on the border of East and West Berlin. They said it was to protect East German citizens, but it was most likely to keep them from defecting to the West. The Wall became the symbol of the Iron Curtain, the border between the East and the West. In 1990, Germany was officially reunified, and the Berlin Wall was taken down.
