For much of the early game, Russia was the closest empire, and the two of us worked in harmony. Multiple research pacts, a Declaration of Friendship. I even bailed them out with some money.
However, their ruler, Catherine, started changing policy. First she puts a city near my border, then screams at me for settling too close to her. Eventually she stops offering research pacts, then has the nerve to denounce me. The main source of friction was that she desired my vast tracts of land. Odd considering her empire was equal in both city count and controlled area, so she had some vast tracts of land herself.
Curse you, Catherine of Russia, and your sudden, inevitable betrayal!! And by that, I mean I saw it coming a mile away and struck first.
While she was still playing with spearmen and archers, I was position tons of Minutemen along my border(she was doing the same... on my border). Then I research Rifling, and upgrade the entire army to Riflemen at once, declare war, and push across the her border. The Riflemen's base strength(25) was twice that of her entire cities'. Within a dozen turns, her capital was surrounded.
Her first offer of peace was extremely generous; several resources, gold, open borders, and about half of her remaining empire. At first I was tempted to accept, but in the end declined. She had no appreciable military left. Her second offer, after being reduced to one final 3-pop city, was a measly 106 gold. I was making more than that per turn.