The Mexicans probably did try. But the US didn't have much of a navy at this point in time anyway. They didn't at the beginning of the OTL ACW either, but they did a fantastic job of extemporizing (I love that word) a brand-new US Navy out of virtually nothing. Props to Gideon Welles and Gustavus Vasa Fox.Didn't the Mexicans try to blockade them in OTL? Then again, I guess that even back then the American fleet was way better than the Mexican one. Still, remember that naval technology is considerably less advanced than in the OTL Civil War.

And naval technology probably wouldn't have that great of an effect on things; sure, the US wouldn't be able to build monitors, but the only places those would be useful would be on the Sabine and Red, or on the Texan coast (no point in sending them to the Rio Grande; Mexico can shift for itself), all of which are fundamentally defensive - and a US-Texan war will not involve the US being defensively minded. Besides, Paixhans guns were installed on ships in the 1840s anyway, so that innovation would be available to the Americans.
If he doesn't, couldn't the French theoretically just support the Italians some more? I forget the particulars - and I'm not within reach of my books on the topic to double-check - but I was under the impression that it was the mild anti-clericalism of the Piedmontese (which set off the Catholics in France) plus the attack on Napoleon himself that swayed him away from further supporting Italian ambitions in Venetia and/or the Patrimony of St. Peter. Could provide a different context for von Bismarck's German unification, anyway, or even Prussian expansion sans von Bismarck.das said:It might be better if he doesn't, and Napoleon is less obsessed with Italy and more obsessed with the New World as a result (maybe have him unsuccessfully shot by a visiting American/Mexican?).
...but that's a butterfly, and as such is relatively irrelevant.