How and Why did Belarus, Ukraine, and Russia develop as separate nations

My understanding is that prior to the mid second millenium, East Slavs were largely one nation, so to speak, with a more or less unified language (insofar as that was possible in those days). However, by the rise of the modern Russian state, Russia had clearly fragmented into Russian, Ukrainian, and Belarusian peoples. How did this come about and why?
 
Maybe PLC occupation of Ukraine and Belarus gave impetus to drift away from OTL Russians.
After all Russian culture came from Kiev Rus.
 
Wasn't the earliest Ukrainian national awakening centered around the Catholic or Uniate Churches or Greek Rite Catholic Churches or something similar to that?
 
I think distance is probably the biggest factor. Peasants in the Ukraine didn't really have contact with Peasants in Russia. The more surprising thing is why did Russia hold together?
 
I think that the separation became inevirable once the crux of "Ruthenian" (East Slavic) civilization moved to the Volga principalities, such as Moscow permanently. I agree with what's been said regarding Poland and Lithuania helping to shape the separate Belarusian and Ukrainian identities, but I'd add too the Tatar/Mongol influence over the Great Russians also contributing to the wedge being driven too.
 
Tartar invasions were the key factor. Before then you had a series of Principalities with a common religion and linguistic variation going south to north but basically a common culture.
After the invasions you had the North-West as semi-independent vassals. Ukraine as a wasteland and Belarus taken over by the Lithuanians. Fast forward 500 years and you have cultural splintering.
 
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