Are you sure starlifter? I was playing as the English not long ago and I was pretty buff at least mighty, the russians split.
I've split civs before, and I was
always inferior to them on the power scale. In particular, I was playing OCC conquer the world about a year ago, and did it alot.
If you were mighty, then someone had to be supreme. Here are the ratings:
Pathetic - Weakest
Weak
Inadequate
Moderate
Strong
Mighty
Supreme - Most Powerful
Coupled with your statement, this means that you (The English) were Mighty, and the Russians were Supreme. You had 6 or fewer civs surviving in you game, and when you took the Russian capital, civil war swept its Empire, and a new AI was born. And the Russians were hostile to it, and eventually went to war with them, at least at first.
If you have a .SAV wich is saved just before the capital is taken, and you had a Split with a Supreme rating (or 7 civs), post it so we can have a looksie.... but with the Split tests I've made (not exhaustive though), plus the wisdom of Smash & others, I'd pretty much conclude you must be inferior and need an open civ slot for civil war to have a chance. I've never ever split a civ when I'm supreme.... and like many others, I've killed hundreds of AI capitals as supreme

....
