No.  If you think like a ruthless military strategist, and don’t mind civilian casualties it’s VERY doable.  Read the history of Sun Tzu and how he made his name.  Here’s how I would do it.  You have to go unconventional.

From a conventional standpoint – the basics have been covered by other answers well enough. Even without a military, The United states has a pretty well armed civilian population, and a HUGE amount of territory, hundreds of millions of people.  We can raise a militia of 50 million well armed men very easily.

Invade, occupy, conquer the US?  Unlikely in the conventional sense.

But you could raid, sack, and burn down the US and take allot if you had the resources of a major nation (Russia, China, India, Pakistan, England, France, maybe Israel or Iran).

Step 1 – Q Ships, Light ballistic Missiles, Nuclear Electromagnetic Pulse.

A  Q-ship technically is a WWI thing – really just an innocent looking freighter or civilian ship with enough concealed weapons to surprise and destroy pirates and submarine attacks on a transport convoy.

So first you get some really innocent looking freighters – there are thousands of them today.  You put some old school SCUDs, or other relatively simple Ballistic missile systems in the hold pointed up.

You arm them with the best atomic warheads you can get a hold of.

You get as close to the US shores as you like – and you fire those missle up into high altitude above the US – probably multiple times from multiple ships, ideally east coast, west coast, gulf of mexico, and great lakes.

Nuclear electromagnetic pulse

The idea is to make lots of those red circles over as many cities as you can.  And a couple of orange and yellow ones for good measure – say 10 or 20 missile shot up into high altitude of 20 miles above major population centers.  Plus a few higher and wider if not as strong, just for good measure.

You could do the same from Disguised satellites as well, but it would be much harder.

Result of Step 1:Nuclear electromagnetic pulse (EMP)

Without entering US territory, with a handful of freighters launching nuclear warheads, detonated high in the atmosphere, you can saturate the US with electromagnetic pluses.

So What?  Well, given the state of the US infrastructure, as I understand it from my career engineering it:

• All consumer electronics will be dead.  All transistors get fried by an EMP.
  • Your car won’t start – electronic ignitions have been standard for decades.
  • No Cell phones
  • No Internet
  • No TV
  • No Refrigeration – food will start to spoil in days
  • No Microwaves or electric stoves. Really hard to cook food, unless you have a grill on your porch.
• All US infrastructure will be damaged and/or inoperable.
  • Limited Commerce / Money.  Most of us use credit and debit cards.  With out internet, you are stuck with the cash in your pocket.
  • No Electricity – the power plant control, transmission, and distribution systems are not shielded, and it would take weeks or months to get the electricity back on; after undamaged parts are imported from outside the EMP area.
  • No / Limited Water – because the water supply is pumped.  If the pumps have no power, your faucet won’t work.
  • No / Limited Natural Gas – Same pump issue as the water.
  • Limited Fuel – all the electric pumps are dead, so you have to know what you are doing to simply get gas from a gas station, if they let you use tools on their pumps.  Even if you have the cash, and a 40 year old car, would they let you siphon from the main tank?
  • Food supply
    • No cold storage – fresh food goes bad in days
    • No vehicles – can’t transport food supplies
    • Industrial farms and agriculture need  electricity and water.  What happens to the feed lots, industrial dairy, chicken factories, irrigated fields?

So imagine waking up one morning, and everything is silent.  No Cars.  No Heat.  No Air Conditioning.  No Phone.  Your Cell phone is dead.  No radio or TV.  Nothing turns on.  Even your watch stopped, and your LED Flashlight won’t work.  The food in your freezer is starting to warm up, you can’t take a shower.

And you have no idea why because all communications are out.

And the only vehicles that work are kick start motorcycles and some cars and trucks, the older the better.  All test that have been done  in the last ten years show that some vehicles die, some don’t, some malfunction.  The less electronics in the car, the better the odds it will survive.

Now multiply those problems by 300 million people across the United states.

At best, you have a humanitarian nightmare, because nobody knows what is going on, and our technology and tools have been reduced to mostly the dark ages.  The only things that still works reliably are guns and fire.  And I will bet that looting and crime will be a huge problem.  You can’t call for help.

Consider that after a few days, 300 million people will be confused, desperate, dehydrated, starving and cold in the dark on foot with no way to know what is going on or if help is coming.

And the military will have the same problems.  The Navy ships will be ok – but a few hundred ships need to get here first, and can only do so much, near the coasts.  The cold war era weapons were pretty well shielded from EMP – so most military trucks, Humvees, some of the helicopters, some of the jets, will still work.

But the electricity and water problem is true for the military – as most bases use city electricity and water (They have published studies).  Most of the military battle networks, digital communications, GPS, lasers, smart weapons, Night vision, digital cameras, won’t work.  The Army will basically be stuck with 1980’s technology.  They say some of the digital stuff is shielded enough, so the Army may still have some laptops and digital radios.  But the infrastructure to support 21st century battle space awareness and communications – will be limited.   They will still have some magic, but very limited compared to what they are used to.  Don’t expect to see many drones.

So basically – after Step 1 – which is find a deniable method of EMP attack on the continental US.  Basically you have a massive humanitarian disaster with the US plunged into the dark ages, and the military will have their hands full simply trying to take care of it’s own, and provide as much humanitarian support as they can, while crime, looting, or scavenging becomes necessary for most of us to simply eat.  Not to mention the healthcare, disease, and sanitation problems.

So the entire US population is facing a humanitarian nightmare, and the military has additional logistics, supply, and communications problems on top of that.

Step 2 –  If you are organized and prepared, step 2 is is literally Sun Tzu 101.

You can play hard or soft.  But the recommended course of action would be to take advantage of the chaos and take whatever you want.  You can sack and burn down cities, steal whatever you like, and simply avoid confrontation and staged battles with an otherwise distracted military.

If I was running the invasion, I’d basically send out commando raids to start burning down every major city, as quietly as possible (make it look like criminals), so the Army isn’t looking for me.  You want to burn every soft civilian target available, live off american supplies, and quietly just make things as hard as possible on the civilians and military without letting them know you are there.

And then you are talking dark ages warfare.  Spread disease, use chemical weapons, burn down the cities, burn the crops, destroy or poison the food and water supply (simple as E.coli in some cases).  Steal everything you can.  Even a few thousand troops operating off of Q Ships with follow up support from Military ships, maybe air drop troops just to cause problems.

In a month you could have every major US city in flames, with a the civilian population starving to death, thirsty, and cold in the dark; and a military overwhelmed by too many problems, and probably not enough food even for the military.

Step 3 – Wait.

If you really do it right, you don’t let them know who did it.  Even better, populate your commando teams with third world mercenaries and terrorists…  You have decent odds at starting the dark ages, and covering the hillsides with third world terrorist arsonists and saboteurs…  Spread bio weapons and disease.

And never ever have a single citizen of you country ever set foot on US soil.  Total deniability – make it look like super terrorists instead of a state action.

Step 4 – Do what works best for you.

• If the US collapses under it’s own weight – you could probably pull of a credible conventional invasion on “humanitarian peacekeeping” grounds, with UN support; at least after a few months and tens of millions have starved to death.
• If the US survives, the economy, industry, and military will be in shambles, and you can basically step up as a world power to fill the vacuum left by a crumbling US dependent on humanitarian aid.

Now odds are you’d eventually get caught, and suffer reprisal from the US Navy, which has the ability to bomb most nations into the stone age even without homeland support.

But any target is vulnerable, if you know where to look, and understand science.

Nothing is uninvadable.  You just develop a strategy that exploits the weaknesses of your opponent while avoiding or redirecting it’s strengths.

So the way you conquer a nation with an unbeatable military is to attack it’s civilians, soft targets, and infrastructure using weapons that the military cannot counter, then wait for them to starve to death behind their super weapons.  You can do it with 1960’s technology, never use a gun, robot, or any science fiction.

If anybody sees any flaws or mistakes in my assessment, please let me know.

Thanks, this was a fun exercise.