What was the main purpose of the second world war? did it need to happen?

The main purpose of the second world war was to prevent one country or group to dominate the entire world, which in that case, was preventing a corrupt leader, Adolf Hitler, from eradicating groups of people based on their race, religion, etc. If this were to happen, this would allow one group, the Nazis to overpower the world which would lead to several international problems. However, while this was caused by the first world war, this did not necessarily need to happen. The only reason for this was due to Germany's economically and politically deprived state as a result of a treaty they had agreed to after the first world war. Because of their state, this allowed one corrupt leader to take over the country, supposedly promising a better future for the country and convincing the people that one group of people was at fault for Germany's misfortunes. 

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