There are 33,453 US troops in Japan right now, and there are about 30,000 troops in South Korea. There are about 150,000 US troops alone in Iraq and they aren't able to keep the peace well. Imagine them against North Korea's huge army and potential Chinese backing.
You seriously underestimate their forces or overestimate the US troop numbers. If war breaks out there, we WILL need to send troops. A lot of them.
North Korea's army is large, but it's almost all infantry and what armor and aircraft they have are all hopelessly obsolete (even more obsolete than China's). At first they can do some damage, but they have no staying power. Plus South Korea has a large, advanced military which would be enough to counter North Korea.
China isn't going to invade us...
They're going to help dig the grave for our economy.
Exactly. Something people don't get in the midst of this fear mongering about China is that it has far too much to lose. What made the Soviet Union dangerous was its isolation. It had nothing to gain from the current system, and so had every reason to destroy it.
The real problem is that China and the US aren't really talking WITH eachother as much as they should, they are talking TO eachother far more than they should. That's something I hope will change in the next 10 years, for the good of everyone.