The Difference Between the First and the Second World Wars

The First World War was partially a historical war; the Second World War was wholly a twentieth century war. 

In the First World War, some belligerent nations were motivated by the notion of righting the historical mistakes of the Medieval and Classical Ages. Tsarist Russia wanted to acquire Constantinople in the name of their Orthodox faith. Greece wanted to acquire Constantinople for recreating the ancient Byzantine Empire. Instead of achieving their aspirations, Russia and Greece suffered a decline. Russia was captured by the Bolsheviks, and Greece was badly mauled by the Turks in a Middle Eastern conflict. 

The Second World War had nothing to do with the Medieval and Classical Ages. This war was precipitated by the rise of two Western forces: Nazi Germany and the Soviet Union. The war came to an end when the democratic faction of the Western powers joined hands with the Bolshevik faction, the Soviet Union, to defeat the Nazi faction, led by Germany.