For world war one I, starting it in say the 1890's and last for decades, the unrestricted use of biological and chemical weapons, neutral nations picking a side.
For world war II
The entry of various neutral nations through either invasion or force to side with one side or another.
The Western Allies vs Axis that has allied with Nationalist China VS USSR VS Japan
Unrestricted use of Biological and chemical weapons
A war where Nazi Germany's high command and most of its troops retreat into the Alps and make a last stand there
The West and the USSR go into a shooting war immediately, and the USSR only surrenders when the last commissar is killed
Japan does not surrender
What would the casualties be? How long do you think a war like this be?