I just watched A24's Civil War last night (great movie that you should watch, btw) and I thought it would be an interesting thought exercise. 
For the US, I think it probably over gun rights or a certain president having a third term. I doubt Texas and California would enter into an alliance though.
Both serious and silly answers
	
		
			
		
		
	
			
			For the US, I think it probably over gun rights or a certain president having a third term. I doubt Texas and California would enter into an alliance though.
Both serious and silly answers

