Few notes.
1. China does have an aircraft carrier but it's a very old and out dated model and China does not yet have the tech, planes or training to actually launch and land planes from it yet. So till that happens it's useless.
2. N. Koreas subs are A bunch of old, loud and slow subs that would be decimated by U.S. forces.
3. By conquer do we mean win a war or occupy that country?
4. It wouldn't be the US v "the world". Japan, Israel, South Korea, Australia, Columbia, the Philippines and NATO would all obviously join the US in some grand war.
5. Many other states would try and stay neutral such as Mexico, Brazil and India.
6. It would most likely be Russia, China, N. Korea, Iran, Iraq, Syria, the stains (half dozen countries), Cambodia, Venezuela, Argentina, Bolivia and Nicaragua.
7. Yes Russia, India, China and others are trying to narrow the military tech edge the US has in certain areas such as air power. However the U.S. is broadening it in other areas. Such as the now functioning rail guns and lazers on board our naval assets and how they are being adapted to aircraft. Also helicopters are a huge US advantage as well.