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 Orthodox faith. Greece and other Balkan states wanted to recreate the entire Byzantine Empire in the name of their version of the Orthodox faith. Instead of achieving their aspirations, Russia and Greece suffered a decline. Russia was captured by the Bolsheviks, and Greece suffered badly in the Middle Eastern contest.
The Second World War had nothing to do with the Medieval and Classical Ages. This war caused by the rise of two twentieth century 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. But the issues in European and Middle Eastern geopolitics are still not resolved—this makes a Third World War inevitable at some point of time in the future.