"annexed Hong Kong prematurely" ? I'm not sure what you mean here.
Reports I've read are that China was surprised when the UK got in touch a few years before 97, wanting to start the prep work for the handover of HK. China, it seems, just assumed the UK would call HK a historical legacy whose lease agreement was made with a now-defunct country, and leave it at that without a handover.
Reports I've read are that China was surprised when the UK got in touch a few years before 97, wanting to start the prep work for the handover of HK. China, it seems, just assumed the UK would call HK a historical legacy whose lease agreement was made with a now-defunct country, and leave it at that without a handover.