You can't attack militarily. Wouldn't it be convenient if America just let untold numbers of soldiers into the country, and even gave them free transportation to cities around the county, and then attacked from within? Crazy talk, right?
That's an easy one.
I would use gun nut Americas second amendment absolutist laws against it, and train my operatives right under the noses of all of the Constitutional sheriffs. After i've got 100-200,000 well trained, well armed, with all of the ammo they can get for their weapons.
The fun begins
Starting with political assassinations at every level, with media personalities, and the billionaire class, and their families.
Once this is seen by the public, that the most well connected, well protected are not safe. That creates nationwide panic, and fear, that there is no one in control, and no one is safe.
The rest of the politicians, and their rich donors then head for the airports to jump on their private jets to leave the country, and local law enforcement hunkers down to protect themselves.
All of the gun nuts with their AR15's are to chicken shit to do anything, so they stay hunkered down in their holes, so they are no threat to the operation either.
The President then declares martial law, and thats the end of Democracy, and America as you know it. along with any rights you once had.
That includes your second amendment rights. You don't get to keep those after martial law is declared. So if you intend on keeping your guns, you now can't be seen with them.
The lower 48 is way to big in land mass for the entire military to police, so they have to restrict any movement. Now the entire American economy is gone. People will be starving within 6 weeks. The world is now in an economic depression. China, India, and Russia take advantage of the situation.
Think any of that can't happen?
Think again.