This might be true if the population of your town is 500. But if it is 100,000, the people at the edge of your group have neighbors who are not in your group. They form their own group, and are part of two groups. As the groups get to know each other, they unite... until the whole town of (did I say 100,000?) 10,000,000 get to know each other and are on the same wavelength.
China's million man army doesn't stand a chance against your guns.
There are a couple of wildcards in such a scenerio (assumed to be China's million man army vs. armed American citizens defending their turf'.)
1) The U.S. govt could provide very valuable inteligence support (at least) to the foreign attackers. Especially with the help of the Israelis who have played a strong role in U.S. internal intelligence gathering for quite a while now.
2) Both the Chinese and more recently the U.S. governments conceive of the biggest threats to their position being their own internal populations and there are just to many of us for their tastes in both countries. Especially since as technology moves forward, there is less need for flesh-bots to run the equipment. Such a framing will be even more the case under a 'one world government' leadership and is probably the main driver behind the 'SCN'. A mutual destruction of all combatants would be 'not a bug but a feature.'
