Entente: UK, Germany, United States of Greater Austria, US, Greece
VS
United Powers: France, Italy, Russia, Serbia, Ottoman Empire,
VS
Asian Union: China and Japan
How would this world be led up to and how would the war turn out?
I'm not sure how to get this, but the Entente is almost certainly going to win in Europe. The combination of largest industrial power in the world+most powerful army in the world+most powerful navy in the world is unstoppable. The Asian Union is going to gobble up any European colonies they can get their hands on, but they probably won't be able to extend their each much beyond E Asia. It depends on which side the Asian Union joins. I can see the Asian Union delaying its entry until one side appears to be wining, then taking the colonies of the losing side. That's pretty much what Japan did OTL, and it got big results for them. I think they'd be more likely to attack the United Powers, largely because a) they're less powerful, and b) Russia is one of them.