China might be because the main landers view the Hong Kong people as foreigners and would turn on them like attack dogs. In the USA it would be very different because the military themselves are mostly in support of the people and the constitution. If you order them to disarm the people and attack they will not follow those orders, so then you have a civil war with the people and most of the military on one side and the politicians and a few tyrannical units on the other. Finally, I'm sure you can think of more than one example in the past half century where the full might of the US military was held at bay by disorganized bands of farmers armed with rifles.