Make that shorter. Quiet easy. In OTL WW1 lasted pretty much as long as it was possible. So someone should win earlier.
have it happen earlier. that'll help a lot.
1. war tech is less advancedHow earlier Great War helps getting not so disastrous war?
1. war tech is less advanced
2. less soldiers
3. likely, you have a shorter war
Medicine would be worse, though, and disease was a significant killer.1. war tech is less advanced
2. less soldiers
3. likely, you have a shorter war