Yeah bro the UN and China could totally just take over America overnight. Nevermind the obvious problems like with a US occupation like: 1) Geography: The United States is fucking massive. Even the 13 colonies in the 18th century were larger than most European countries.
2) Gun Ownership: Forming civilian resistance groups to fight an foreign invasion would be really easy. Especially considering that the United States owns 46% of the world's 1 Billion Guns: news.yahoo.com/americans-own-…
3) Logistics. The United States is mostly geographically isolated which makes it costly and time consuming to transport troops and heavy industry to America. It would take months to coordinate such an invasion. Yoh could argue the UN would use Canada as a "starting point"...
..But even this isn't a perfect plan since Canada is also mostly geographically isolated like the United States. It would be a logistical nightmare for both the UN and China regardless to send equipment and troops to conquer America.
4) Defense Pacts: There's also nothing stopping balkanized American Regions from forming defense pacts in order to provide collective defense against an foreign threat. And before anyone says "this would never happen", there would be a strong incentive to do this.
If a single balkanized state gets conquered this could jeopardize the sovereignty of surrounding regions and could lead to further invasions. The fear of being conquered would create a strong incentive for mutual defense even amongst competing states with different ideologies.