Britain ruled Hong Kong (a territory not a country), until 1997 when they turned it over to China. The reason Britain owned Hong Kong, beginning in 1842, is really fascinating. China had a huge trade surplus, because they had so many products Europe was interested in such as porcelain, silk, and tea, which not unlike today, hurt the other countries doing business with them because money was leaving their economies. This is where it really gets interesting. The East India Trading Company from Britain, began growing opium in India, and smuggled it into China illegally. The influx of narcotics reversed the Chinese trade surplus because so many citizens were spending money on drugs versus spending it locally. A war broke out called: The Opium War between Britain and China and Britain won.
Part of the treaty between the two countries was that Britain could lease Hong Kong from China for 99 years which ended in 1997. Based on input from other countries like France, German, and the U.S., they had Britain add to the Agreement that when China did take ownership back of the territory that they couldn't run it like the Qing Dynasty was running China (later termed communism), for a period of 50 years which would end in 2047. It was to be a territory with separate laws that were geared towards human rights. They also weren't allowed to take ownership of any businesses (state owned).
This worked out great for the citizens of Hong Kong as it's one of the most successful cities in the world with the largest number of wealthy people per capita. Then things went south and the citizens of Hong Kong are in an uproar. In March, China attempted to influence laws regarding extradition treaties, meaning Hong Kong would have to turn anyone over to Chinese police whereas before Hong Kong could refuse. The citizens of Hong Kong don't trust China and they're afraid that if anyone said something against communism or China in general, that China would take them and either murder them or send them to hard labor camps. The bottom line is they in no way want an extradition treaty with China. So far five citizens have been killed during the protest and with China threating to send troops, it's likely that more will be killed.
When I look at situations and try to figure out what's going on, I usually look at it through the lens of power, influence, and money. With this in mind, I can't help but think there's more to this story. This is just a theory on my part, but Trump has significantly hurt the Chinese economy in an effort to get them to the table for a trade deal that would work for the U.S. Hong Kong is loaded with wealth. Maybe China doesn't want to wait until 2047 to tap into the extraordinary money supply. If they can say Hong Kong is out of control and that they must impose martial law, it would allow them a foot in the door to take control of the territory and run it however they'd like. They're not worried about a world war over the issue especially since they know that countries like France and Germany are now running from fights and not following through on threats. The U.S. certainly won't go to war on their own regarding Hong Kong. Anyhow, I might be totally off base, but maybe I'm not.