It started after the treaty of Versailles when Germany ‘s land was taken, they were not allowed to have an army more than 100,000, no navy, and had to pay money for damaging France and Britain and to make it worse the Wall Street Crash or Black Tuesday known as Stock Market Crash of 1929. Germany was very weak and the people thought the old Germany would only rise with a dictator to stabilize the situation and re build the army. The place was for Hitler and he had found it and wanted it, Hitler charged all the Germans with strengthening and empowering words until they elected him he did all he promised but destroyed Germany.
World war one was differently motivated than world war two. The countries did not fight because they hated each other in world war one. They fought for one reason that was they had to help and defend their allies. The Germans had to go because when Austria-Hungary declared war on Serbia and Russia declared war on Austria-Hungary. On the other hand the axis powers went into war for land and resources. This can be seen in the invasion of Manchuria by Japan.
In the end, those two wars were the worst two wars of the twentieth century which resulted in millions of casualties because of greed and ridiculous reasons. So, world war one started because of assassination of the archduke Franz Ferdinand and world war two because of the invasion of Poland. The world has benefited a lot from these wars and the most United