One country, two systems
| How the sausage is made|
One country, two systems (一國兩制) was a Chinese political and economic system proposed by Deng Xiaoping in the 1980s to get Hong Kong and Macau back from Britain and Portugal, both former imperial states.
This meant that Hong Kong and Macau were officially Chinese territory, but they would guarantee at least 50 years of strong self-rule in politics, economy and society, which in principle is similar to before the handover to China.
In the late 1990s, Britain and Portugal gave their final colonies, Hong Kong and Macau respectively, to China. However, as both had been influenced by their colonial masters, the powers both made agreements saying that the two cities would have home rule for at least 50 years. This means that China will be able to revoke the home rule from the cities in 2047 for Hong Kong and 2049 for Macau. Since both have developed their own distinct ways of life, it is unknown how it will play out, but it probably won't end well if China tries to reduce or end the autonomy, as both are effectively city-states.
While the one country, two systems principle was well adhered to between 1997 and 2000, when Hong Kong was first returned to China, the 2010s saw a backlash from many liberal Hong Kong citizens as the Communist Party of China tried to intervene actively in Hong Kong's politics. This corresponded with General Secretary of the Communist Party of China Xi Jinping's consolidation of power within mainland China. It hasn't yet been 50 years yet, but the one country, two systems principle is increasingly being undermined.
Around April 2019, China drafted laws which would have allowed for criminal suspects to be extradited to mainland China. Hong Kong citizens were naturally alarmed; massive pro-democracy protests against the law began shortly afterwards. Due to a timid response by the Beijing-friendly Hong Kong government, protests continued throughout the year, culminating in local council elections in November 2019 where pro-democracy candidates won in a landslide. 
COVID-19 briefly halted the scale of the protests. Rather than let the protests simmer down, however, China elected to draft an even more draconian "national security law" to apply to Hong Kong, that essentially gives Beijing the right to quell any dissent in Hong Kong against Beijing rule, using China's authoritarian law system. This law, passed into action on June 30, 2020, was designed to apply to anyone, anywhere in the world, with Beijing directly setting up its own national security agency in Hong Kong. "Serious" national security cases will be tried in Chinese courts, by Chinese judges. Merely unfurling a flag advocating Hong Kong's independence is enough to get you arrested under this law.  The response from Western nations (for now, at least) was some rather toothless finger-wagging.
So, as of June 30, 2020, "one country, two systems" can effectively said to be dead.
- "The Hong Kong protests explained in 100 and 500 words", BBC News, 2019 November 28
- "5 Takeaways From China's Hong Kong National Security Law" by Emily Feng, NPR, 2020 July 1
- "How China killed Hong Kong's pro-democracy movement in less than a year, while the rest of the world were too weak to help" by Bill Bostock, Business Insider, 2020 July 1
- "China’s draconian security law for Hong Kong buries one country, two systems", The Economist, 2020 July 1