On a timescale like this the creation of ideologies favorable to decentralization (or many many other things) is really no problem. Employing it within the context of the Russian Empire seems like a bad idea, so likely we're talking about either a revolution or a military conquest (or both).
Perhaps the easiest (though not the best for Russia) is a different WWI where Germany's victory is more complete and a more thorough domination of Russia results in this decentralization. That's a lot of territory to dictate terms to, but then the POD doesn't specify the borders of "Russia," so perhaps we're talking only about European Russia.
ITTL let's say Germany's more illiberal tendencies die in the crucible of victory and they promote democracy in forms of ever increasing equity to all of their allies. Eventually, it gets to the point where Germany is more like first among equals than dominating power, and all of the Russias are an important bloc of voices in the alliance.
Alternately, you could have Russia go revolutionary (either due to a war or just mounting social unrest), happen upon decentralization on their own as a system worth trying, and develop ties with a democratizing West over a period of years. You could have the alliance form in the aftermath of a collapsing Germany, giving up its satellites, or you could have a less oppressive system in place that simply welcomes Russia once everyone's cleared their heads of the idiocy of the imperial build-up of the 19th century.