Since China has long-range decades-long plans, they no doubt had a timeline or script for a take-over of HK.

The USA can do almost nothing to help HK. One of the few non-military options would be economic such as a trade war but we are already in that. Perhaps that was one of China's options - that they decided to move on HK if there was a trade war.

Hong Kong is on their own much like Hungary was in 1956 and Czechoslovakia was in 1968. The USA wanted to help the people in those nations but was powerless due to the timing and risks. The people in HK must try to act calm and cooperative in order to avoid the heavy hand of Beijing.

Will China also go after Taiwan soon?