Damn the AI can expand quickly on Emperor. I've learned that if I don't see at least two good city spots closer to me than anyone else I need to immediately prepare for early war, no matter what civ I'm playing.
I just had a game as Brazil, I went scout, warrior, settler. I got my Liberty worker and then my Liberty settler very early due to popping a cultural ruin, so I had two settlers very early in the game, something like turn 25ish (I also chopped to hurry it). Even still, I got completely boxed in. Completely. Spain took a city spot in one direction, and Denmark took another. I literally had nowhere to expand. 2 settlers at turn 25ish, maybe turn 28, and I had literally not one single expansion spot. Oh sure, there was some random land around, but no other luxury resources and therefore I couldn't afford to expand.
That's the thing with civ 5, you can have tons of land around you and still get boxed in because unless you can grab lux's, you can't afford to expand.
So anyway the reason I got roflstomped like that and lost the game at turn 30 was because those expansion spots were about halfway between myself and my neighbours.
Lesson learned - if you have expansion spots that are much closer to you than anyone else, go ahead and peacefully expand. Even then you should rush those settlers out like your life depends on it. If the only luxury resources you can see are halfway between you and a neighbour, literally DON'T even try. Rush spears/archers instead, because you're going to be taking an AI city on that spot.