I was just tossing off ideas, in no coherent order, so let me clarify:
1. First off, I’m assuming that for the purposes of this discussion, that I have a large (Army sized) mechanized force with equivalent air power. As there aren’t too many countries that can afford this…well, it’s all theoretical anyway, so just assume.
2a. I know what my Strategic Objective is. And that I am able to position my forces (preferably Canada and Mexico, with a few Marine-type invasion forces, about Brigade strength, concealed in RO/RO Car Carriers off the East and West Coasts) undetected.
2b. Knowing my objective and plan of attack, I have accordingly properly shielded my own forces from the EMP, and have sufficient stockpiles of food (combat rats.; YUM-MY!) medicine, fuel, parts and ammo, as well as the prime movers to transport them.
3a. A 20 megaton nuke, in a low orbit over the northern hemisphere, will completely fuck all civillian-grade electronics, including the factories that make not only civillian electronics, but military electronics as well. Even the hardened ones.
Cars. Commo nodes. Power nodes. Hospitals. Water Distribution networks (whaddaya think controls the pumps and valves? Buh-Bye New York and L.A.!) Airports and their aircraft. Kiss them all goddbye.
3b. Detonate in early January. With the control mechanism to transport petroleum off-line, the northern 2/3 of the U.S.A. will freeze and/or starve to death in a week (I’m assuming a “Liebensraum” motivation for my attack) unless the military responds quickly to enact emergency relief measures.
Only the military has the resources to ensure order in the resulting chaos; food, medicines and shelter. Given the dispersed nature of our civillian population, this will scatter the military to a fare-thee-well. Large transprt aircraft will be busy flying relief supplies to major urban centers; troop formations will be deployed in a peace-keeping, urban pacification mode. Mechanized combat formations will be stripped of personnel to man and administer refugee centers. Helicopters (cargo variety) will be used to move relief supplies from nodes to where they are actually needed, while sophisticated gunships, with 9:1 Maintenence/Operation ratios, will quickly be grounded when the supply of sophisticated electronic and mechanical parts run out (no factories to make them!).
4. Give two weeks time for the American military to fully deploy in a relief mode, then invade. Take everything from the Mississippi River to the Rockies, Canada-to-Mexico. Put your newly-acquired serfs to work in the fields come Spring Planting, and reap the harvest.
When the forces guarding the cities come after you, deploy your Marines to take the urban centers and destroy their remaining technological manufacturing base in hit-and-run raids, then dally in the U.S. Force’s rear to keep pressure off of your formations in the midwest. This supports you, as the U.S. Forces simply cannot ignore Brigade-sized elements in their rear areas. They either:
a. split (weaken) their forces to guard their rear while attacking you, or;
b. ignore you to secure their rear, or;
c. ignore their rear to come after you full strength, with the consequence that they’d get corn-holed in the ass by Force Recon style deep-strike light infantry, striking from bases in the newly-captured urban centers (N.Y., D.C., Charleston (S.C.), etc.,)
5. The Nuclear Counter-Strike ICBM’s are Strategic weapons; you just don’t (actually can’t!) reprogram them to hit a mobile, tactical formation. For an apt analogy, try swatting a fly with a grenade launcher, inside your own home. Doesn’t work, and just opens up your home for more flies. Besides, would you really want to nuke your own “Bread Basket”?
You can “Win the Hearts and Minds” of the people by supplying them with food, shelter, and medicine (in exchange for forced labor, I mean, er, “gainful employment”, of course! :D) once the American military runs away to fight your main force in the midwest.
6. I cannot emphasise this enough: mobility, Mobility, Mobility! Positional warfare is suicide for my invasion force, as then they are vulnerable to ICBMs! Take key terrain features (my aforementioned “Mississippi River to the Rockies” statement), as they are easier to defend from. The U.S. Military is woefully underequipped/unprepared with “Engineering Assets”. Take key mountain passes out west and bridges on the Mississippi River; blow the rest to deny their use to the U.S. Forces. Make them come to me on the terrain of my choosing.
Note to the Militarily Impaired: “Positional Warfare” and “Defensive Warfare” need not be the same thing; they can be, but not necessarily are by default. “Operational Art” can leave you a fully mobile force capable of defensive engagements in a fluid battle.
These are still just rough ideas; I’m assuming Strategic and Tactical Initiative throughout, which isn’t guaranteed (there are some pretty sharp General-rank people in the military who could, quite concievably, turn the tables on me and my forces).
I’m trying to put as much logistical strain as possible on the military by crippling the civillian infrastructure; they either ignore the “starving masses” to fight, or hamper themselves with humanitarian relief to the point that their combat effectiveness is degraded. Hopefully to the point that they are no longer truly combat effective.
If they ignore the devastated civillians, my purposes are served in several ways:
1. There will be less people to pacify. The old, the sick, the weak will die off, and the survivors wll be too demoralized/weakened to effectively resist.
2. The individual soldiers comprising the combat formations coming after me will either be so furious that they’ll act foolishly heroic or be so demoralized as to be practically zombies.
3. My relief forces will have thousands, if not millions, of civillian hostages with which to bargain. Ask Johhny Soldier Boy if he wants to continue fighting if he knows that his wife and children are quite possibly alive and well, and all he has to do to be with them in my new forced labor compounds is to lay down his rifle and walk away.
ExTank
“Maybe Not Quite As HArmless As You Would Like To Think” 
I have a video at home with all the major tests, narrated by Captain Kirk. One of the tests that fascinates me is (I forget how it was named) one where they were testing the effects of altitude on the blast area. In one firing, they used a relatively large weapon (for the time) at an altitude of 1500 feet and got firestorms and a good radius of blast damage. A school bus and a jeep that were there for test purposes had all the paint burned off of them and rocked in the breeze. Next, they used a much smaller weapon at an altitude of 500 feet and got some sort of weird blast wave that crumpled the bus and jeep and threw them several hundred feet.