I read on the game chat "I think Mexico supported the south because the mexican-american war took their lands"
Well..wrong.
Mexico supported the North, during that time Mexico got invaded by France, the excuse that the french had was that Mexico didn't pay their debt, so France invaded Mexico.
One of the objectives of the french army was to quickly destroy our army and then invade USA and support the south, but Mexico defeated France on that war (commonly known as "Cinco de Mayo")
There is a dedicated letter from Abraham Lincoln to Benito Juarez (mexican president), the north was thankful of us beating the french army.