Your assuming occupation is out of the question because of its sheer size. And your wrong. Most of the population lives along the coast. Most of the inner countryside is desolate and hardly anyone lives out there. The US could just ignore everything other then the coast and still occupy the country of China.
Were still capable of a FULL victory. And not all wars require you to occupy something.
The country of China is assbackwards in how it runs its economy. The population is for the most part poor. While there are a few uber rich. This is how China has operated for millenia's and time and time again rebellion has refreshed the country but it always lands back in the same boat.
China is starting to catch up. But only in certian fields. It doesnt have the capacity to implement technology into its Military fully. And probably never will. China prides itself on size. Which means most of its own military funding is locked up in upkeep rather then expansion.
The US wins its wars because its finally realized that for its Armed Forces to be a true threat it has to be a threat at all times. Which means funding new projects that give the US the edge. The ColdWar is what brought this revelation about.
Vietnam was far more of a success then you might realize. The biggest reason for its downfall was public opinion. It was painted as a war of bloodshed (like wars before hand had never seen death) and that Soldiers were murdering villagers for the hell of it (it happened, but it wasnt the first time in war) WW2 saw entire cities flattened...cities meaning civilian populace. Meaning Civilian casualties. Civilians dying during war is nothing new, and can be seen all the way back to the first days of what we classify as War.
Afghanistan hasnt sapped us of any power. Infact its testing us and giving us the ability to adapt to a different form of guerilla warfare we werent expecting. This can only be good for our Armed Forces as it widens the scope of our troops adaptability and our use of technology.
What seems to people as sapping is how much money was spent. Because it was in the billions. But people forget that whats 1 Billion now was Millions 50+ years ago. Because the worth of money only increases with time. We think were spending WAY more money then ever before. Which is and isnt true.
Iraq hurt us, but only because currently only 1+ million of 300 million serve this nation. Making our ability to place our military might limited. But that came and gone. Iraq is now back in the hands of the Iraqis. Only 6 short years after invasion.
Because of the fierce fighting and antiinsurgency tactics and strategies developed and learned in Iraq. We can now turn back to Afghanistan and adjust them. The main reason Afghanistan has taken longer to overcome. Is because Iraq was so unpopular and pressure was far greater for us to resolve it.
People for the most part still believe Afghanistan is a good fight. So the pressure to succeed there like we did in Iraq isnt as high. While we want it to be over. We still remember why were there in the first place. Meaning were still willing to take our time with it. Unlike Iraq where we had no real clue why we were there. And wanted every reason to pull out.