One country, one system; otherwise only wars and civil wars can partition a country and undermine its sovereignty and territorial integrity. That has been the norm of modern states, beginning with early modern Europe and slowly spreading around the world, up to this day.
It is against this almost universal understanding of state sovereignty that we can appreciate how extraordinary "one country, two systems" really is, a unique historical experience in a time of peace and plenty, one we scarcely have the vocabulary to describe.
That is why, with their Western-centric experience, practically every foreign observer predicted, wrongly, that China would simply swallow up Hong Kong after 1997. That's why mainland officials seconded to Hong Kong have consistently complained about the city being a difficult or impossible book to read and understand. Standard communist or patriotic propaganda enables them to do their day-to-day job, but not to truly understand. Likewise, many activists from our democracy "rah rah rah" crowds come to believe it's the British legacy, which laid down core values such as the rule of law, that distinguishes us from the mainland, oblivious that it is really "one country, two systems" as codified in the Basic Law and honoured by Beijing that helps preserve them and sets us apart.
In a brilliant undergraduate thesis from Harvard, my young friend Nicholas Gordon examines how the Hong Kong and Macau SARs punch holes in the traditional concept of sovereignty, a binary notion of "you either have it or you don't". In reality, all kinds of cities, territories and even countries have quasi-sovereignty without full independence. And so it is with our city, which functions as a separate entity in practically all domestic and international arenas except defence and foreign affairs. Under such circumstances, we would have developed a separate identity, or otherness, even without the democratic movement. In this sense, Horace Chin Wan-kan, the intellectual godfather of the "nativist" movement, is not entirely wrong when he calls Hong Kong "a city state". Beijing, under the Basic Law, sets it up that way. What is dangerous is his call for autonomy, which invites mainland interference or even a takeover.
The tragedy of "one country, two systems" is that its success proves to be its undoing. Hong Kong is drifting apart, and Beijing must reverse that. Allowing for full democracy is one solution; another is to search deep into China's own rich historical experience, such as the old tributary diplomatic system, to accommodate diverse societies.