I mentioned this in another thread about how long WW1 can be prolonged. As far as I can see, a prolonged Western Front will lead to increased unrest amongst the armed forces and the civillian populations of all the participating nations. As we know, there was the October Revolution in Russia, the French mutinies, the collapse of Austria-Hungary into nationalist revolt, the German Revolution and plenty of near misses in both France and Italy.
So as far as I can tell, Leftist revolutions happening within the warring nations is not too unlikely, hell it could even be seen as the only logical conclusion if neither the Central Powers or the Entente can break through and a diplomatic end cannot be reached.
The problem here is that whichever side collapses first, the other side will take advantage and go for a big push. The only way to prevent this is to have everyone go off at the same time, which is rather difficult

I think a POD preceding the war, where the Second International maintains some unity in opposing the conflict would work. It would allow for the increased co-operation that would be required for orcheastrating revolutions in several countries at once.
Anyway, my predicted end result would be something like this.
So this is what I imagine would come of this big ol' revolution. Britain btw is not part of the USSE. Suprise suprise, they refused to join

The pink countries have applied to join but don't quite cut it yet.
So, how could this be achieved with a POD no earlier than 1910?