EMP is the way to go. Tanks, Helicopter Gunships and Jet Fighters require constant maintenence utilizing sophisticated mechanical and electronic parts.
While these [electronic] parts are EMP “hardened”, they are not EMP proof, and thus the possibility of a partial immediate reduction in my enemy’s warfighting capability.
The industrial base that produces those sophisticated parts, however, is wide open to EMP attack, as is the national electric power grid, communication’s networks (not the military’s, though) and just about every damn thing from medical equipment to grain silo elevators have a chip in them somewhere.
With an EMP strike, you have a good chance of seriously effecting several disparate components of the American economy, from food production/distribution, health and human services, transportation, communications, oh, the list goes on and on…
If your invading army is able to slug it out for about 30 days, then the American military juggernaut will grind to a halt all on its own accord as fuel and parts run out, and no replacements are to be had.
Launch your attack in the first third of winter, when petroleum demand is at its highest. That way either the military has to take all the fuel reserves to fight the war, or millions die of cold in the northern states.
Millions in the cities will die anyway, or else there will be massive population displacement, as the urban centers are unable to feed themselves once the food distribution network and trabsportation network is disrupted.
This social unrest will sap the strength of the American military as forces either have to be diverted to support the refugees, or instill martial law and supress Food Riots.
The biggest wildcard is the “militia”.
Before launching my ground war, I’d first wait until a full, 100% gun-ban was implemented, with door-to-door siezures and massive meltdowns of confiscated guns.
I don’t want those pesky partisans running around behind my lines stealing fuel, food, medicine, guns and ammo, and blowing up anything they can’t carry off.
If my invasion force was adequately concealed (in other words, complete Strategic Suprise and Initiative), I’d EMP the states and wait until the American Military is fully deployed and dealing with the social disorder before crossing the border.
This would have U.S. Forces too dispersed to effectively defend during the outset, and it would be a good start on leeching the military’s supplies and logistical network prior to the commencement of actual hostilities, with the added benefit that a deployed/operational logistics network is fairly vulnerable to attack. Sneak in some light fighters disguised as civillians to attack truck depots, fuel depots, distribution nodes, roads, railroads and bridges of all sorts.
It wouldn’t be a cakewalk, but it’s not as difficult as some people think.
The biggest flaw I see, logically, is sneaking a large mechanized force, undetected, to within striking distance of American soil. The Mexican Army is only good at finding cerveza (Mas Tequila!), and I just don’t see the Canuck’s doing something like this.