In a game where I was Roman in a large continent, the Chinese, a powerful civ, were on the north beyond and at the mountains. I was in the grasslands. There was a small yet long mountain range desert to the west and then a gigantic jungle and a U-mountain range. (continuation of the Chinese mountain range.)
In the Jungle and a bit near it in some grasslands and plains next to the desert lived the Indians. I destroyed or captured most of their cities.
Then Peace enveloped the region. The Indians set up many cities in the desert, and I was doing the same thing, even though most of the desert was unproductive. During the next War with the Indians, I was able to take the desert cities.
Did the Indians know there would be Saltpeter in the desert? Perhaps. It almost looks like they're trying to get the saltpeter.
It seems like a really intelligent AI, but maybe they were just trying to get more land, just like Russia in your game?
Maybe the Russians thought you were weak? Maybe they had many more units than you? Strength is also a factor.
Usually civs don't attack you unless they're (A) trying to get land just for the heck of it, or (B) militaristic. If they are Furious with you or if you, in the units that they see (remember they don't know your strength unless you show it) are weaker than them, this aggravates the situation.
