Answer:
All answers in bold. My previous answer didn't save, so I'm making a new one.
Explanation:
The United Nations was formed in 1945, by the Allies of World War II. The five permanent members of the UN are China, France, Great Britain, the USSR, and America. The USA and the USSR did not remain allies after the war because of the following ideological differences.
a) Economic: USA - Free Market USSR - Communism
b) Political: USA - Democracy USSR - Dictatorship
Each country decided to build up its military power, in case the differences should ever come to war. Proliferation of nuclear weapons and other weapons of mass destruction created a conflict known as the Cold War. The leader of the USA was Harry Truman at the outset of the Cold War, while Josef Stalin was the leader of the USSR.
At the end of World War II, the Soviet army occupied most of Eastern Europe, while the Allies occupied most of Western Europe. The agreement at Yalta established a framework that divided Europe into two Spheres of Influence. The North Satellite Treaty Organization or NATO, was founded in 1949 to create a common defense for Western Europe and North America.
Meanwhile, Eastern Europe would be under the Soviet Sphere. In response to NATO, the Soviets created the Warsaw Pact which stated that the nations of Poland, Czechoslovakia, Romania, Hungary, Bulgaria and East Germany would protect each other. Nations under the umbrella of the USSR were known as satellite states (I'm not too sure about this one, you might want to double check). Great Britain's former Prime Minister, Winston Churchill, said in a 1946 speech that an "Iron Curtain" separated the East from the West.