For starters, Hong Kong’s 31,000-strong police force lacks the manpower, and its officers may refuse to use deadly force; after all, there is a big difference between firing rubber bullets at a crowd and potentially murdering civilians. This means that China would have to deploy the local PLA garrison or transfer tens of thousands of paramilitary soldiers (the People’s Armed Police) from the mainland.
Hong Kong’s residents would almost certainly treat Chinese government forces as invaders and mount the fiercest possible resistance. The resulting clashes would mark the official end of the “one country, two systems” arrangement, with China’s government forced to assert direct and full control over Hong Kong’s administration.
With the Hong Kong government’s legitimacy destroyed, the city would become ungovernable. Civil servants would quit their jobs in droves, and the public would continue to resist. Hong Kong’s complex transit, communications and logistics systems would prove easy targets for defiant locals determined to cause major disruptions.