Funny since the un made taking land illegal the amount of involuntary border change due to invasion has dramatically collapsed. Do you just think that is a coincident? The un may not stop an invasion but it greatly reduces the motivation to invade thus China would not want to invade.But the US does exist. Luckily for us its also in America's interest to have us in this region of the world so our alliance has a long way to run. If the US didn't exist or decided we weren't worth protecting we would get invaded in a matter of weeks and there's no force on the face of the earth that would stop it even if it could. Your stark raving mad if you think the UN or Nato would step in.
plus the current UN isnt the end point of globalism anyway. Its only the start.