I just triggered 'Foreign Intervention' for my first time, playing as the CSA, Fall 1862.
QUESTION: am I supposed to be able to attack Mexico City? The French won't move there and neither will the CSA.
The CSA gets a British 'Army' in Canada (about two Divisions), a British Army in England (powerful Army plus two Corps), a British Fleet, a French Army in Mexico (about two Divisions), and a French Fleet. The USA gets a Mexican Army under Diaz.
So, the US immediately attacked Manassas and lost almost 200,000 troops (compared to my entrenched 75,000 losses). I've still got Manassas but have not much AotP left, but then, neither does the US.
And for some odd reason, Diaz and the Mexican Army decided to help the US put down the Sioux Insurrection; now they're somewhere around California (main force) and Kansas (cavalry?). The Mexicans also split off what is now an amphibious force that is off the coast of TX / LA.
My Canadians are stalled after taking Detroit MI and Plattsburg NY, my Brits are resting after taking Boston and New York. My French are besieging Chihuahua. The US Navy is in trouble (in addition to the Foreign fleets, I have Fort Monroe, with an additional Coastal Artillery unit in a Division commanded by Huger the Artilleryman - they pound any ships trying to enter or leave the Chesapeake).
I thought that the game would pretty much be over, but the US National Morale was actually increasing (until the US lost its Army).