Modern Poland has a long history of being either a territory or client state of Russia, but at one point in time this was not the case. Poland is a successor to the Polish-Lithuanian Commonweath, a large state that stretched from the Baltic to the Black Sea. This ethnically diverse federation was one of the largest and most populous states of Europe in the 16th and 17th centuries and was unique for its checks on monarchical power and relative religious tolerance. Meanwhile, what would become Russia, the Grand Duchy of Moscow and later the Tsardom of Russia, was still a minor and isolated state. They wouldn't begin to supplant Polish power in Eastern Europe until the late 17th century.
So how do we prevent this from happening? Published works of alternate don't give many great examples. Polish-wanks aren't exactly unheard of in the genre (see Pez's "The Drowned Baby Timeline" for an online example), but most have their points of divergence set in the 20th century (I w nastepnym dniu by Maciej Lepianka) or rely on alien space bats to fix the problem (Adventures of Conrad Stargard by Leo Frankowski). If we are going to keep Poland strong and Russia weak, we need to have the point of divergence earlier in our history.
To be honest I do not feel confident about my knowledge of either nation's history to feel I can craft a proper alternate history. That being said, my brief reading of Wikipedia makes me wonder if the 17th century would be the most fertile time for our change. During the Time of Troubles, Russia was at its lowest point and Poland even occupied Moscow for a short time. If things had been even worse for the Russians in this period (more deaths from famines, no stability under the Romanov's, the Cossacks don't rebel against Poland, etc.) perhaps they would never recover fully and thus be confined to the northwestern corner of our timeline's Russia.
So what happens next? Will Poland expand across the Urals into Asia, eventually reaching the Pacific and colonizing America the long-way around? Will rump Russia become more Scandinavian in outlook? Would more Orthodox Christians recognize the Pope? I fear I may stray to far into parallel history if I try answering these questions. Let us know your thoughts in the comments below and if want to submit your own scenario email me at ahwupdate at gmail dot com for a chance to be featured on the next What If Wednesday.
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Matt Mitrovich is the founder and editor of Alternate History Weekly Update and a blogger on Amazing Stories. Check out his short fiction. When not writing he works as an attorney, enjoys life with his beautiful wife Alana and prepares for the inevitable zombie apocalypse. You can follow him on Facebook or Twitter.