Thing is there is no realistic scenario where the US would be invaded by anything....
The US military is currently a good 10 years ahead of any other military in tech, with the closest being russia and France. All other countries in the world rely on outside arms purchase and are de facto dependent on the furnishing country for their wearponry. Even China is lagging way behind, didn't they just test their FIRST stealth plane a few months ago?
The other true factor besides tech is the quantity of gear (tanks/planes/boats.... even if lower tech) and in this aspect too, the US have a big step forward any country (well maybe not russia there, dunno). I mean even if you have 1000 chinese littleman with rifles... they ain't gonna do much against a bomber out of bullet reach...
Lastly, experience... well the US is in every fight...
On the numbers though, if you recall japan, they were many many less than the chinese pre WWII, yet they did manage to cover most of asia... granted, japan is larger than north corea, but the only factor that enabled them to do this was still the military tech that was way ahead than any other countries arround them.
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