After the first world war Germany was very harshly punished in the Treaty of Versies which is why Hitler took power. His plan was to expand into the 3rd Reich taking over all of Europe, Asia, and Africa, to give the German people living space.
Hitler was able to greatly expand the German Empire, but ran out of resources to protect the land he had and continue gaining more, which he wanted to constantly do gaining him many enemies.
As Hitler began loosing more and more battles, he realized that there was no stopping the inevitable, his forces were captured, killed or unable to return and help, and the British had become unstoppable. Hitler left with his wife and killed himself and he, leaving the rest of the German people to come to a decision about what to do. Germany unconditionally surrendered and ended the war in Europe. Now all that is left of WWII is in museums.