One of the cats claimed the comfy box for itself to sleep in, while the other one had to kip on the cold hard floor in the corner. So I picked up the one on the floor and placed it in the box with the other cat. The other cat didn't like this and started growling in a 'get the hell off my turf' kind of way. So I started stroking them both at the same time and they soon forgot about their beef and focused their attention upon on how nice strokes feel. I stroked them so much that they began to settle and then started cleaning each other before falling asleep.
This technique could be used to create world peace. Instead of telling two opposing groups to be nice to each other, don't mention the issue at all and just be nice to both of them, to the point where they focus on and appreciate your constant niceness so much they forget about their worries and their strife, and passing on the nice vibes becomes the natural thing to do next.
Note: I'm not advocating that the UN ought to assemble Donald Trump and the leader of ISIS in a room together, and then start stroking them until both men feel compelled to bathe one another.