Why Did WWII Happen?

WW 2 happened because Hitler was out of control. Most of the time when nations contain their wars with in their borders other countries do not get involved. Once things start affecting others the more powerful nations of the world feel the need to step in and protect the less powerful nations. Hitler was very close to invading Britain and it seemed the US wanted to help a friend but it was probably more of a concern with how close Hitler would be to invading the US at that point.