So, you want the USA to take on the planet, eh? Well.... Here is the best route to take.
Step 1: Control the ocean. Use the vast power of the US navy to quickly cripple the rest of the worlds fixed wing carriers as there is only a handful, and two of which are either laid up unable to be used (lol russia) or still in training mode (China). Plus a few more are small, middle aged smaller vessels that are used more for defense and patrolling than a real threat. Follow this up by quickly hammering the rest of the ships, everything you can hit quickly and either send to the bottom or into dock for repair. You wont get everything, but you don't need to. Just enough that what's left finds itself vastly outgunned and unwilling to get into a straight on fight. Once thats done, just sink everything. Bring global shipping to a halt. Doing this would isolate most of South America and Australia, and heavily impact Britain and the rest of the EU.
Step 2: Heavy air strikes. Hit refineries, power plants, rail centers, everything. If you can cripple their civilian communications as well, even better. Use bombers and ground based special forces to bring it all down. Government centers would obviously have backup systems, as would medical centers. BUt the rest would be SOL. If you manage to succeed in doing this step as well as the initial phase of step 1 in a tight timeframe, you could send entire populations into a mass panic. After the initial strikes, just keep at it. Eventually the US would just run out of missiles, but by that point the damage done would be immense, and repairs would be measured in months if not years. Massive long term blackouts + no incoming supplies from around the world and you just eliminated most of the world from the war as they would no longer have the means, and possibly the will, to fight the US and internal issues would be taking up most of their resources.
Step 3: Play the waiting game. The US has everything it needs to hunker down, defend it's coasts, and strike when the opportunity arises. War of attrition the rest of the worlds navies, keep global commerce via the sea lanes dead. Small embers of unrest that are already in many countries would turn into all out civil war. Countries with long standing dislikes towards each other would be at each others throats. Now you are no longer taking on the entire world, just the big dogs that manage to hold it together. South America is effectively out of the fight, as is most of Africa. China will probably be busy dealing the with inevitable fight between North and South Korea plus the potential of a very dirty war between Pakistan and India. Russia is busy annexing old territories to cement it's power. Europe is holding on, but also bolstering itself for a potential conflict with a renewed monster in Russia. Isreal is taking on all comers defending itself as the middle east erupts into a bloodbath. Mexico and Canada SHOULD have been beaten quickly in the initial phases, and won't work as invasion points. The US itself is now defended by two large oceans, and an inhospitable landmass to the north, and the rest of the world by now is dealing with other problems.
Step 5: Begin forcing surrenders and armistices. South American governments go first, as they will not have the manpower or ability to argue. Promise relief supplies and support to rebuild, install puppet governments. Kick the insane amount of available manpower they have into high gear building the essentials to keep people at home happy and keep the war going. Pick your battles carefully, use control of the oceans and air to continue to hit high value targets. Eventually more and more countries will call it quits just to end it. Mostly smaller countries, third world, and isolated. But as you do so, you begin denying valuable resources to those that want to keep fighting.
Final step: Starve the rest out. With world trade at a standstill, the middle east erupting, and conflicts everywhere, supplies will run out sooner rather than later. Fuel, food, and basic necessities become rationed, and then dry up. Use plants and supply lines to fuel unrest to cause more problems. World militaries, instead of using supplies to fight the US, are now using supplies to fight each other and keep their own territories in line and running. Millions (if not billions) are going to die at this point world wide. Mass starvation, succumbing to the elements, genocide, it's going to be insanely bad. Eventually more and more countries will either collapse entirely making them easy pickings, or flat out give up the fight. Russia, China, and Europe last the longest as their standing armies are enough to prevent a land war, but with their sea and air forces depleted they know it's a stalemate. And with much of their territories in ruins, they have enough troubles at home and go with cease fires.
End result: A draw at best. Much of the world is now under nominal US control, but mostly in terms of the Americas and Africa. Europe, China, Russia, and their territories are dealing with the aftermath of billions of people being dead and their infrastructure being in ruins. The middle east is now a lost cause as most governments have fallen to various factions, and nobody else has the time, energy, or resources to go in and clean up. It will take decades to rebuild everything, and there will be lasting resentment for probably the next few hundred years over the war. Instead of hundreds of individual countries, you now have 4 massive powers in the world that don't trust one another at all, beginning a massive cold war and arms race. The world never fully recovers, world peace never happens, and the next war literally becomes the final war as there won't be anything left standing by the end.
Mind you, if any of these steps go badly, the US is immediately on the defensive and will probably lose outright. And all of it is based purely on everything going right, and the first two steps being accomplished quickly with minimal losses. But if the US wanted to try to win, it's the best shot they have and the US Navy is key to all of it. So all in all... odds are the US wouldn't win. They sure as shit would make a mess of some people, and the death toll would probably end in the hundreds of millions at best, but it's not a fight the US could realistically win without some seriously good rolls of the dice.
