The answer happens to be much longer than this, but here's a short answer.
In the 1930's after WWI, Germany was in horrible poverty and was being torn apart day by day. They were so desperate for help, they started listening to a man named Adolph Hitler, who was blaming Jewish people on Germany's poverty, and he eventually took over power from Germany's former government. Adolph said he would help make Germany better, which he did for a while, but then he turned into a monster, opening up things called concentration camps, where people are sent to be killed for no apparent reason. He started a race called the Nazis, who were horrible people who killed anyone who wouldn't convert to being a Nazi. World War II is about countries fighting against countries who were with Germany. USA were part of the people who wanted to stop Adolph Hitler, and finally succeeded in 1945, when Russia and USA invaded Germany and took over power. Adolph Hitler ended up killing himself after knowing that the Allied Powers (people against Germany) were coming after him. This terrible war lasted from 1939 - 1945.