One country, two systems
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One country, two systems (一國兩制) is 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.
The "one country / two systems" arrangement was set up when Britain handed over rule of Hong Kong in 1997. The arrangement 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. In Hong Kong, the system was designed primarily to achieve the dual objectives of preserving a prosperous Hong Kong while securing China’s wider national interests of territorial sovereignty. 
Decline and death in Hong Kong
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 be said to be dead in Hong Kong.
For now, the "one country two systems" arrangement is still alive in Macau. Macau's system lacked many of the British-inspired democratic elements of Hong Kong's arrangement, and Macau's government was much more closely tied with the Chinese government, having had elements of China's communist party integrated into the island's government even 33 years before independence. Thus, very little of the unrest that sprung up in Hong Kong during 2019 was seen in Macau. 
One country, Xi's system
Since June 2020, the mainland has continued to tighten the noose on many freedoms Hong Kong previously enjoyed such as freedom of speech... bringing the island closer and closer to the rigid authoritarianism of China, as dictated by Xi Jinping.
- Fearing any bit of pro-democracy power in the Legco, in late July 2020, Hong Kong authorities disqualified 12 pro-democracy candidates from upcoming elections. A day later, the elections were postponed altogether (conveniently citing COVID-19 as a reason). 
- In November 2020, Beijing issued a new directive to disqualify lawmakers it deemed "unpatriotic". Conveniently, four lawmakers that were part of the 12 candidates barred from the postponed election were deemed "unpatriotic" enough to be disqualified almost immediately. This resulted in pro-democracy representatives resigning en masse, declaring that the legislature was no longer a legitimate political platform. Democratic Party leader Wu Chi-wai, stating the obvious, commented that China had "totally given up on ‘one country, two systems’ in Hong Kong". 
- By January 2021, China was cracking down further on dissent and accelerated using the "National Security Law" to crush opposition voices. On January 6th 2021, 53 pro-democracy activists were arrested under the guise of the law.  By March 2021, Hong Kong made its 100th arrest under this law.  High profile people arrested under the law include media tycoon Jimmy Lai and activist/politician Joshua Wong. 
- In early February 2021, Hong Kong ordered schools to adopt a more patriotic curriculum (even overhauling the kindergarten-age curriculum to include propaganda) and advised teachers to report any breaches of the national security law. 
- In March 2021, China implemented a major overhaul to Hong Kong's election system, designed to eradicate what little political opposition was left. The proportion of directly elected representatives in the Legco was halved. The "election committee" that chose Hong Kong's chief executive was significantly expanded in size with many additional unelected seats, with many seats that were previously held by directly elected district councilors abolished. All candidates were required to be vetted for political loyalty to China. 
- Having satisfactorily reformed the legislature to crush all opposition, in April 2021 a new "election" date was announced for December 19, 2021. 
In response to the increasing authoritarianism in Hong Kong, in late January 2021, Britain opened up a visa scheme for Hong Kong residents with a British National (Overseas) passport, offering a possible fast track for these passport holders to UK citizenship. To no one's surprise, a significant number of Hong Kong residents took advantage of that offer, or chose to emigrate to other nations, including Taiwan.
- "Hong Kong Under One Country Two Systems: Promises and Realities" By Chris Yeung, Visiting Fellow, Center for Northeast Asian Policy Studies, Foreign Policy Studies Program, Brookings Institution, June 2001
- "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
- Mainland Chinese vistors Macau stays cool, Scmp.com, May 2, 2021
- "Macau: China's other 'one country, two systems' region" by Sophie Williams, BBC News, 20 December 2019
- "Macau: Is Hong Kong's neighbor China's 'poster child'?" by Melissa Sou-Jie Brunnersum, Deutsche Welle, 2019 October 28
- "Hong Kong bars 12 opposition candidates from election", BBC, 2020 July 30
- "Hong Kong postpones elections for a year 'over virus concerns'", BBC, 2020 July 31
- "China forces ouster of Hong Kong pro-democracy lawmakers, quashing opposition" by Shibani Mahtani and Theodora Yu, Washington Post, 2020 November 11
- "Hong Kong opposition lawmakers all quit after four members ousted" by Lily Kuo and Helen Davidson, Guardian, 2020 November 11
- "National security law: Hong Kong rounds up 53 pro-democracy activists", BBC, 2021 January 6
- "Hong Kong Makes 100th Arrest Using National Security Law" By Natalie Lung, Kari Soo Lindberg, and Chloe Lo, Bloomberg, 2021 March 2
- "Hong Kong media tycoon Jimy Lai charged under national security law" by Jadyn Sham and Helen Regan, CNN, 2020 December 11
- "Hong Kong pro-democracy activist Joshua Wong arrested under national security law" by Jessie Yeung and Eric Cheung, CNN, 2021 January 8
- "Hong Kong Imposes Sweeping Pro-China Curriculum on Schools" by Kari Soo Lindberg, Bloomberg, 2021 February 4
- "Beijing cuts Hong Kong's directly elected seats in radical overhaul" by Emma Graham-Harrison, Guardian, 2021 March 30
- "Hong Kong: What is China's 'patriot' plan for electoral reform?", BBC, 2021 March 30
- "Hong Kong's Delayed Legislative Elections Set for December" by Associated Press, 2021 April 14
- "Hong Kong residents now eligible for special UK visa", BBC, 2021 January 31
- "London Is Emerging as the Destination of Choice for Affluent Hong Kongers" by Portia Crowe, Penta / Barron's, 2021 March 15
- "Is Taiwan Next?" by Sarah A. Topol, The New York Times Magazine, 2021 August 23