I'll chime in with a different take. China would fill the vacuum but it wouldn't do so with military power, it would do it with its economic power.
Based on my limited knowledge of Chinese history, the Chinese are not the conquering type.
Basically most things would stay the same.
I think China would be only too happy to fill the void with economic AND military power. However, the biggest that would happen is that Europe and free Asian nations would greatly step up military spending.
I think in many ways better (not all), definitely cheaper for the US--but probably not a huge lot would change. Other nations (UK, France) would step in to fill much of the void, thereby shouldering the cost that is currently shouldered by the US.
I do think overall the US spends too much time playing world soldier. It's like after dicking around in WWII and not helping its allies (as it should have done as soon as WWII started) it has swung the other way.
Agreed. We discovered the problems with isolationism; it leaves evil free to take the rest of the world one bite at a time, so as to be unstoppable when it does get around to us. Now we're discovering the problems with being the world's policeman, namely that it's very expensive and it breeds resentment.
I'd love to see the USA become more isolationist, starting with removing our troops from all foreign bases except those with ongoing conflicts (South Korea comes to mind; they are legally still at war with North Korea) and using that money to develop American-made rapid heavy transport and forcible entry capabilities. I'd also like to see America move from defending whomever needs defending to making explicit and totally bilateral defense treaties with individual nations; we agree to defend you and you agree to defend us, including maintaining proportional armed forces and spending proportionally on the military. Other than that, if a nation thinks it needs a US base for its own protection, let it sign a bilateral defense treaty and pay our associated costs. I think we'd find that most of our bases are there not for defense, but for the money they bring in. I'd also like to see us put out a new Monroe Doctrine if you will, declaring that Afghanistan is our last nation building exercise. Henceforth, if you attack us we will simply isolate you by destroying your ports, energy infrastructure, and military capability, making it difficult for you to maintain and project force to threaten us again.
I'd also like to see the US move out of the UN and start another such organization open only to truly free democratic republics, similar to NATO. That will not happen though because we have a big organizational advantage in the UN with our permanent veto power. I do think that our share of the UN expenses needs to be drastically reduced, and I think that if the UN wants us to do the heavy lifting defending a member nation then it can damned well hire us. (Granted, to a degree that actually happened in the first Gulf War to liberate Kuwait, but I believe that's the exception.)
With respect to #3, I'd like to see America with protectionist tariffs once again and instead of "free trade" where we agree to tie one arm behind our back, sign only symmetrical trade agreements based on mutual restrictions. Where labor costs cannot be equalized, a system of tariffs would equalize costs, and trade imbalances would be addressed via tariffs as well. If our exports to you significantly exceed our imports from you then you may unilaterally reduce your tariffs; if our imports from you significantly exceed our exports to you then we will unilaterally increase our tariffs, with the aim being to balance trade. I think if we completely cut off trade then we all lose, but as it standards today we're losing, and generally intentionally out of some misguided sense of fairness.