It started with the attack on Pearl Harbor in Hawaii
http://50thingstonotknow.blogspot.com/2006/12/world-war-iii-almost-started-in-1995.html
In simplest terms, the main cause of World War two was World War one, consult your local history teacher for more information.
The UN was started in 1945, just after World War II.
It was the assasination of archduke franz Ferdinand that directly started world war 1
It started when ............................................ was assasinated
It was Germany's invasion of Poland that began World War II.
The assassination of Franz Ferdinand
The assassination of Archduke Ferdinand
World War One started.
Franz Ferdinand
It was the assasination of Austrian Archduke Ferdinand