China took control of HK in 1997, and it runs until 2047, when it's to be absorbed into China formally. Today is operates as a Special Economic Zone and Autonomous City Province within the People's Republic of China. It's sovereignty is China.
In practical terms the HK agreement is in effect a gentleman's agreement and little more. The courts have already shifted final legal authority to Beijing. This is controversial, as Britain claims it's court still has equal authority, but in practice the British courts have lapsed. The international authority over HK matters is as much as China allows, which is to say none in practice, a little bit in pomp and form.
The realities on the ground are more profoundly tilted toward China. Some 6,800 PLA troops are barracked in HK and the New Territories and at least 20,000 more are in Shenzhen (the mainland immediately adjacent to Hong Kong and the New Territories, and as many as 50,000 can be mobilized within a couple days if need. It is probable that the 6,800 man garrison has been reinforced, through rotation in, but not out as tours end -- an easy and quiet way to add force by simply extending service (it would hardly be noticed by HK locals). Chinese TV has shown armored vehicles roaming Shenzhen and nearby. So PLA move is possible.
However, this will probably be handled by police forces rather than the army:
https://www.scmp.com/news/china/military...nniversary
Some 12,000 riot police have been mobilized in Shenzhen and under going training. These are the ones most likely to augment the HKPD. I have been told by friends in HK that the police already include many Mainlanders; they can tell by the accents that they are from Guangdong and not HK. Also some police are refusing to wear badges, which is suspicious. And new tactics, including point plank rubber bullet firing (I can link footage) and taking out individual protesters and using much greater force are earmarks of Mainland tactics and not regular HKPD.
The way I see this playing out is a crack down. It will be by a heavily reinforced HKPD with the storming in of perhaps 12,000 additional riot police. The PLA will take a back up role, providing heavy weapons if needed, but otherwise taking over duties such as harbor policing and border control, to free another couple thousand HKPD for operational work.
If they crack down, they will haul off those they deem as leaders to the mainland for trail and imprisonment. They will likely open some reeducation camps modeled on those in Xinjiang for the Uighers and other Muslims. I see young people taking the brunt of it. The autonomy will continue to exist in name, but the politics will be restructured so that it is effectively a fully internal compartment of the PRC, just with HK face.
HK is already very unsafe for western business, as China kidnaps anyone they want and hauls them to the mainland. The extradition law is mostly a formality, as the HKPD does the surveillance for them already. Then a team comes in from the Mainland and takes their target. It is strange, but true, that they let the Triads go, but worry about booksellers. Electronic eavesdropping and theft of computer information has to be assumed. All the web is Chinese and using Chinese equipment. It is a foolish westerner who thinks there is any barrier.
CHina has already pressured businesses to fire employees who participate too loudly in the protests. Cathay Pacific tried to be neutral, but came under the ire of the mainland and a boycott because "neutrality" is seen as anti-China. But this same pressure, firing of people, students losing school enrollment, is starting to be in effect. This will ramp up, to make the cost so high to protest that few will. And those that do have a gulag to look forward to. The social point system of the mainland will also be implemented in HK, making it difficult for those deemed disloyal to move around.
China looks at the crackdowns in Myanmar and Thailand and how the West didn't do anything. Western business is uninterrupted in Bangkok, as if there never was an uprising and suppression. They expect the same will happen in HK. The West wont act, although some will get on a pedestal and proclaim this a great struggle, not really caring the horrible sentence anyone taking on Xi and his legions will face. The Chinese probably calculate correctly. Intervention will not lead to any change in the world reaction. Maybe some lofty words, but businesses will continue to be dependent on China for supply chain and rare earth materials. (I hope we get smart on that and realize we are depending upon the biggest military and political threat to the US ever)
Personally I have been telling my friends in HK that the protests need to end. If they don't they will be absorbed into China a generation earlier, and they will be subject to the social point system and the repression of China. The limbo status they want to end with clarity is the best situation they can hope for. With that limbo they have one generation, or 20 years to find a way out. But I am afraid the protests have a life of their own and they wont stop until the force of China takes them down.