The US owns most of the GPS satellites currently orbiting in space. By disconnecting the rest of the world from those satellites, the world will be blinded when compared to the US. The US will then follow up with the systematic destruction of every naval fleet on the planet. Given the fact that the US navy is in every ocean and is the most powerful in existence, this wouldn't be as difficult as people think. With complete control of the sea and air, every nation not connected to the Americas becomes irrelevant in the short term. The US would then launch two ground invasions (Mexico and Canada). The US doesn't need to conquer all of central and south america, just establish a strong buffer zone. The first few hundred miles into Mexico should be all the US needs to stop any invasion from the south. Canada is a little trickier. The US would need to conquer all the major cities and the coastline to ensure that any invasion from the north is stopped quickly. Given the size of the US army and the fact that Canada is cut off from the rest of the world, this is not an impossible feat. With the continent secure, the US will begin bombing raids on major industrial centers, nuclear power plants, dams and other major infrastructure, and any shipyards. Doing this can knock out several key players ability to wage war. And by controlling all shipping channels around the world, many nations who are not completely self sufficient will under go a series of resource shortages (Europe is very poor in natural resources l, China gets all its oil from offshore drilling, and several places on the planet rely on trade to actually feed themselves). And given the US' resource capacity and how long it will take the world to recover from this initial assault, the US could continue this for about 50 years or so.
But assuming several nations don't surrender or try to make a deal with the US, the world will recover. Industrial centers will be moved, areas with resources will become the new contenders, fleets will be rebuilt, and so on. And their in lies the weakness of the US. Although the US could defeat any enemy and attack anywhere on the planet, they cannot invade and occupy the entire world. The US doesn't have the manpower to pull it off. So the world will keep throwing fleets and aircraft at the US, and no matter how many the US defeat, more will follow it. Eventually the US fleet will be pushed back, unable to keep up with the flood of enemies that just keep coming until the US navy is forced to go on the defensive, surrounding the Americas in a blockade to defend the homeland. The world will build more fleets and troop transports to try and invade the continent. Most will fail, but not for long. This stalemate could last for about 30 years or so.
Of course the world would win. The sheer number of people would swarm the U.S. in a number of weeks. It's physically impossible for one country in the world to fight against all the others. Any country would be easily conquered by the rest of the world. Although the U.S. has the best military in this case quantity is better than quality
If for some miraculous reason every single nation in the world decided to invade the USA simultaneously then of course the World would be victorious, you'd have missiles flying in to every corner of the country , invisible drones would take out the countries infrastructure in a day, who will America retaliate to, they'd be annihilated in a matter of days.
But that will never happen.
The USA Military could not be beaten, the weakness is American political leadership. Political leadership has been America's military weakness since the end of WWII.
Many Americans consider surrender as a first option, while others believe that America is not worth fighting in defense of. The "give me liberty or give me death" belief in America is viewed by many as radicals that want to keep their guns.
You have to know that Americans are not all 100% Americans, if the world wants to destroy America, it will start right from inside, and I'm not even talking about the Chinese coming over, remember countries like Canada, well they too have a military as well as many other countries, and Japan being the technology geeks they can probably come up with a drone that can take out people like playing Halo. This will be so easy, America is not as powerful as we think, it's the media that make look powerful.
"Shall we expect some transatlantic military giant, to step over the
ocean, and crush us at a blow? Never! -- All the armies of Europe, Asia
and Africa combined, with all the treasure of the earth (our own
excepted) in their military chest; with a Bonaparte for a commander,
could not by force, take a drink from the Ohio, or make a track on the
Blue Ridge, in a trial of a Thousand years..." Abe Lincoln.
I believe that the entire world's army would be swallowed up by America's large landscape. That the U.S military is the largest and best in history, I also believe that any military unit would be destroyed swiftly by U.S defenders. The air force would dominate the sky's, and the navy would make invasion by sea almost impossible. The Abrams tank would dominate the battlefield also, not because of it's superiority in armor but because of it's tracking system the tank commander could immediatly send your location to all the other tanks, you now have 14 tanks looking for you and all the Apaches in the vicinity are also after you, your survivability just dropped to 0.
The U.S Army's third Corps is garrisoned in Texas being able to secure at least some of the region before reinforcements arrive.
I would also like to add some facts about the size of our military. ( If we were being invaded by the world I am sure that most of these numbers would double.)
Budget(2014): 526.6 Billion
Total Air-power(Fixed-wing and Rotary): 13,683
Oil Production: 8,500,000 gallons per day
No one would dare invade the US, as an invasion is a face to face confontation, without the power of A- bombs or drones or cruise missiles or mine. Invasion is face to face combat, and no one would dare. The problem is that we now have a globalist minded politico, not an american political mind. The globe is jealous of us and is perfectly willing to destroy the American mind. America is the worlds Jesus. Crucify us world, we dare you.
The United States is the world's largest economy, and has been a leading superpower within the global community for the last century. The spirit and drive of the American people is strong and they are resilient. The Spirit of 1776 will continue onto generations to come and into the future.
Our military out performs any other military out there. Even if we don't outnumber them. China has more tanks, cool. China, may I introduce to you the A-10 Thunderbolt II. AKA "the tank killer, silent death, the flying tank, etc." Some other country has the most advanced anti-aircraft weapons available, cool. We have JSOWs, HARMs, and MALDs. MALDs are decoys that distract missles, HARMs track radar stations using the radar they need to fire their missles, and JSOWs can target a group of ground targets and destroy them all. All of the other countries' outnumber us in every way. The U.S. has the most advanced Air Force. Without air superiority, the rest of your forces will suffer unimagineably large losses. Large numbers of ground forces are no match for the ordnance our Air Force can unleash. No one can say that we couldn't control the skies, F-22s in one versus many engagements with F-15s have scored over 200 kills with no losses. To top that off, not a single F-15 has ever been shot down, ever. One lost an entire wing, and the pilot was able to land it. All of this points to one thing, you can't stop the U.S. military. Oh, and one last thing, with some of the most relaxed gun control in the world, anyone wanting to invade won't have any idea who is going to start shooting at them at any moment. So, with that being said, I have no doubt that the U.S. is not a counrty that can be invaded.