The Allies wanted to invade Japan's homeland, but that would cost them too many lives. So the Allies told the Japanese that they must surrender or else they'll drop their atomic bomb. The Japanese didn't respond so they dropped it. This killed about 75,000 people. Three days later they dropped a second atomic bomb which killed more than 70,000 people. This forced the Japanese to surrender.
After World War 2, the UN was made to protect the members against aggression. Its base is in New York. There is a General Assembly of many people. Then there was the Security Council made up of its five permanent members (Britain, China, France, US, and the Soviet Union) plus six more members. The Security Council had the real power.
Hitler realized that there was no way for him to survive. So, he married his long time companion, Eva Braun. The next day they both committed suicide. The German military then put forth their unconditional surrender which President Eisenhower accepted, but due to his unexpected death, President Truman witnessed their long awaited victory.
For Hitler to make Stalin join his nonaggression pact, he promised to give him Poland after he conquers it. He succeeds and despite France and Great Britain's appeasement, they declared war on Germany.