Dr dre
The South China Sea does not belong to China, any more than it does to all the other nations that border it such as Indonesia, the Philippines, Vietnam, Brunei and Taiwan. A number of which are subject to a direct threat from bases established on disputed Islands annexed by China. That why the nations under threat are seeking protection from the USA and other nations against what is intimidation and coercion.
Your assertion regarding hypothetical Chinese bases on the borders with Canada, Mexico, the Caribbean and even Cuba is ridiculous. These nations are not seeking defence assistance from other nations because of a direct military threat, and there is no potential stranglehold on trade between these nations and any other country in the world. Commercial and Military vessels of all nations are fully entitled to travel wherever they wish in International waters. Naturally they will be shadowed because some will be capable of aggression and intimidation. Even the Russians and the Chinese pass through the English Channel without let or hindrance. Some do not even announce their intentions by radio and other vessels must be redirected to stay clear of them.
Do not take for granted everything that you read regarding US capabilities in an election year. Everyone wants their piece of the pie when it is served up. You can also be assured that if the time comes to act, the USA will not act alone.
I see that your location is : The World - is that an aspiration?
IG