90% of the Chinese population uses the American dollar as their currency. If China called us on our debt, we'd simply remove them from the national banking system and their money would literally be worthless, causing their own economy to collapse in a heart beat. It's why they never call us out.
And no, they're are not stronger than us, militarily. The US Navy, alone, is bigger than the next 5 largest navies, combined. China has a supposed 600 million + land force, but only about 2 million of those are actual trained fighters. The rest are "reserves" that the government can call upon if the country ever comes under direct attack, but do you really think they could effectively supply such a force. US and NATO would simply need to blockade China, and their military would literally starve to death. Our economy could recover from something like this; theirs could not.









