St Petersburg Event: 1908
POD is that whatever hit Tunguska in 1908, instead of flattening an isolated wilderness destroys the capital of Imperial Russia. Among the tens of thousands of casualties are Tsar Alexander, his family, most of the central bureaucracy and many members of the Duma as well as the lion's share of the Baltic Fleet, stationed at Kronstadt. This has three major effects:
1: Germany is quick to take advantage, under the guise of "restoring order" and almost immediately annexes the Russian part of divided Poland then moves into Russia proper. Although protesting formally France and Britain do little because Russia is indeed in chaos largely because of effects 2 and 3.
2: None of the Big Three of the Bolshevik movement, Lenin, Stalin or Trotsky are in St Petersburg at the time of the disaster and they also move quickly to take advantage of the turmoil, jump-starting the Russian Revolution. However they have two major stumbling blocks they didn't have in OTL. The Germans and...
3: At the time of the Event Russia was the most deeply religious nation in Europe. Many take the Event as a Sign from God. Mobs of the devout begin roaming the countryside, intent on purifying Mother Russia of "sinful elements". These soon come into conflict with both the Germans and the Reds.
This chaotic situation goes on for over a year with Germany adding the Baltic provinces to her territory and the Reds and the "Soldiers of God" contending over the rest of the country until a fourth force emerges from the remnants of the Russian government and army with Prince Georgy Lvov, who managed to survive the devastation but had been badly injured and in hiding., as it's figurehead
With an "official" Russian government having become a faction Britain and France become emboldened to put pressure on Germany to cease it's invasion. Having bitten off about as much as he's able to digest Kaiser Wilhelm agrees but retains the Baltic provinces and Poland. Britain and France then begin funneling money and supplies to the Tsarist forces although not troops for fear of pushing Germany too far.
However prospects for a stabilized Russia are destroyed when the heretofore disorganized groups of religious fanatics are united under the banner of "Holy Mother Russia" by a man claiming to be the monk Grigoriy Rasputin who says he survived the destruction of St Petersburg "by the grace of God".