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Let's say Confederate Colonel John T. Pickett's dueling affair illustrated right here ends up far worse. (Say the fight with Bennett escalates where both he and some Mexican employees are shot and killed)
Pickett is arrested; Mexico demands some crazy indemnity; the CSA ignores the demand; Mexico declares war in 1861.
What happens next?
I assume France has to get involved to either broker a quick ceasefire or tacitly support the Mexican government.