So Russia has proof it was Ukraine, and Ukraine has proof it was Russia.

So how about this.

PUT BOTH leaders of both countries on the spot, saying that whoever is proven to be found as lying by a committee of experts in a PUBLIC TRIAL, will have to either back up their bags and go back to Russia, or give up that part of the Ukraine to Russia.

The beauty of it is, whoever is lying will either be exposed as being so, OR will have to pull out of the bet and will face further condemnation and doubt throughout the world. And there is nothing like a babies brain splattered all over the run away to hit home.

Simple but very effective and the only person that would not think so is the person who knows they are lying.

Its a win win.

Proof must be conclusive, or its null and void. Even if one of the leaders goes back on their word, they'll face so much negative PR it won't be worth either of them doing so.