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World War I is a completely separate war from WWII; most of the governments of the original war had changed, from the Italian Monarchy giving power to Mussolini to the collapse of the German monarchy and the creation of the Nazi Party, to the collapse of Russia and the rise of the Soviet Union. The reasoning for WWII's start was the loss of land the Germans had thanks to the Treaty of Versailles, which had ended WWI; the Germans initially just wanted their land back. WWI's start was due to the assassination of the heir to the throne of Austria-Hungary, by a Serbian nationalist. The Austrians planned to invade Serbia, which the Russians didn't like, so they attacked the Austirans, and the Germans didnt' like it, so they attacked the Russians, and so on. But yes they were two completely different wars.
Allied Countries, WWI
USA
UK
France
Russia
Italy
Japan
Serbia
Allied Countries, WWII
USA
UK
Soviet Union
France
China
Axis Countries, WWI
Ottoman Empire
Germany
Austria-Hungary
Axis Countries, WWII
Germany
Italy
Japan
Austria
Romania
Hungary