Although it is not enough to simply say that Hitler was the cause of WW2, he was a major part of the reason that the war broke out. This is really because of his personality and how the past such as the Treaty of Versailles had affected him. At GCSE level, you need to be able to link different factors together.
Look at this powerpoint to revise how Hitler got to power and what he was promising to the people of Germany:
Once he was in power, Hitler began to make drastic steps. His aims were to destroy the hated Treaty of Versailles, to rebuild the lost glory and power of Germany and to prove his abilities as a leader.
Very cleverly, he started small and worked upwards. He began simply, such as leaving the League of Nations and increasing his army, though he only increased it a small amount. But by 1937, he was acting in a far larger way once he thought that he could get away with it. Use the link below to see a sheet which lists some of his main actions before 1938.