They'll just claim illegal immigrants represent an "invasion." They'll use that as casus belli to invade Mexican territory, and then simply claim occupied lands as spoils of war. They'll also be able to make a practical argument, "well Mexico clearly can't manage the border regions. Look at the anarchy there!"

I don't see MAGA trying to seize all of Mexico. They wouldn't want to take the heavily populated core around Mexico City, that's for sure. But the more lightly populated border states? That would be much more tantalizing. Also, they could justify intervention in the border states as "protecting American investments," referring to all the factories US companies have built right across the border.

Relocating an entire industrial plant is hard and expensive. One of the quickest ways to "bring manufacturing back to America" is to simply move the border to encompass already existing factories.

Canada would be a lot harder to justify, but casus belli can always be found. It just takes one person fleeing to Canada to escape charges for getting or performing an abortion. Suddenly Canada is "harboring fugitives." Or attempt to relitigate the US/Canada border disputes of the 1800s. Or just claim fentanyl is being smuggled in from Canada. Etc. A casus belli can always be found or manufactured.