Guess I will post what happened to me, too, then.
Playing as America, I have two goals- Realize completely Clay's American System, and epitomize the feeling of Manifest Destiny in the American public.
I quickly encounter my first obstacle with the America system- I have very few workers. Two or three is not many for such a big nation. God news, though- all of America is already hooked up railroads. And I accomplish part of the first goal.
To accomplish the second goal, I look south- Polk was nice, but didn't he realize he could have had so much more? Sure, Texas and Southwest is nice, but Mexico would keep problems from cropping up in the next 100 years! After a small buildup, war is declared. The two cities to the immediate south of the border are taken of the first turn with minimal losses. Over on the peninsula, three infranty manage to capture both city with no losses.
I push further and further into Mexico, and eventually reach thei capital. By now I have a few dynamit guns built, and put them to use. Generally innefictive. I order the attack- unit after unit slams into the city's defenders, and I lose many before the city is in my control. Cuidad de Mexico? WTH is that? Promptly renamed Mexico City. Mexico is killed off a few turns later.
Central America has the Panama Canal, an extremely important strategic thingy. They die quick. I was surprised.