They need to find trustworthy allies, move their best troops close to the border but out of artillery range, and prepare for an all-out insurgency war after the regular army takes a bite out of the invaders but is then forced to retreat and melt away into the population.

This is how Iraq defeated the United States.

It would also help to goad Trump into attempting to take on Mexico and Canada at the same time instead of one after the other. Same way Hitler could not fight Britain, the US, and Russia all at once.

While all this happens it is up to us fighting in the belly of the beast to stab this war of aggression in the back, opening a third front inside the US itself. Note that we will have allies in that fight and that will help with things like supplies and equipment.