The Brits had a lease with China on Hong Kong and it expired. It was not a hostile takeover. There was no question of defending it from the Chinese. The UK did get some concessions from the Chinese on Hong Kong's status, which was advantageous for China as well.
If such a defense becomes a reality, I hope we would have a better commander than MacArthur was in the Philippines in 1941.
The US defense of Taiwan would boil down to threatening the PRC with a nuclear attack should they try to take Taiwan by force. Just as Clinton and other Presidents have done, the US Seventh Fleet would play a major role acting as a trip wire.
The real question is what will Obama do when the rubber hits the road. Will he signal weakness allowing the PRC to move or will he send the message to the PRC that any such forcible takeover of Taiwan would lead to a war with the US. There is economic leverage, but that only goes so far.
I was in the PRC just before the took over Macau. They had a huge clock near the Great Wall ticking down the days, hours and minutes when the handover would take place. And now that the Olympics are over, the PRC will increase the pressure on Taiwan to capitulate and to isolate Taiwan more from the US, including trying to dissuade us from selling arms to Taiwan.
No, the lease was for the "New Territories," not Hong Kong. I worked in the New Territories in about 1998. They have very close business relations with Hong Kong proper, so it would not have been easy to separate them.
The Brits were in no condition to defend Hong Kong against a Chinese invasion. I suppose they figured they agreed to the best possible deal.