The Reforma Wars are a short, decisive victory for Juarez. Thus there isn't much damage and Mexico is still able to keep paying foreign debts.
When the South secedes, it doesn't act like a complete idiot diplomatically and plays its cards right, bringing England and France into the fray early, enough for the US to have a shortage of gunpowder as they haven't finished the process to make it on their own.
Desperate, Lincoln asks both Russia and Mexico for help. Mexico, hoping to united the people against a common foe and gain either some land or an ally who won't war with them in the north, declares war against the CSA and especially France, citing the Pastry War as an example of European meddling in North American affairs.
Thus we have one interesting world war:
USA, Mexico and Russia vs. CSA, France and England