I just made a huge blunder.
I decided to put both Roosevelt and Washington in my game to see what would happen if there were two leaders from the same civ in the game (that's right, I've never tried it before - I normally just use "random" opponents, and it never overlaps civs when you do that). It put them in the game as blue (the original color) and yellow (Egypt's color).
As the game progresses, Mongolia settles aggressively right next to my border, so I declare war and go off to fight them.
I raze the border city and then march through the jungles to go (eventually) take the next closest city. In the meantime, there is a bunch of empty land where his city used to be, and the blue Americans send a party up to settle it before I can get to it.
Right after I take Mongolia's next city, I notice a huge stack of blue Americans moving towards their new city. I'm thinking "great, they are going to declare war and attack me." So I start moving units to my closest border city to get ready for the invasion.
Sure enough, a few turns later Roosevelt declares war on me. I keep sending units to that one border city, but he never attacks. The guys are just sitting in their city...
A few turns later, all of a sudden on my OTHER border, which isn't defended at all, the YELLOW Americans cross the line and attack me. WHAT?
Oh, that's right. Roosevelt is YELLOW - NOT BLUE!
So I got caught completely off guard thinking that Roosevelt was his normal blue color and I had actually been preparing to fight Washington the whole time, completely oblivious to
the guy who actually declared war on me that was coming right at me!
I just saved the game and shut it off at that point. I wasn't prepared for this at all, so I'm going to think about it and approach it later.