I don't think you can:
- China has the nuclear weapon, a space program, allies and no ethics or politics to slow them down.
- nobody won the cold war, and Russia was way less economically viable than China
- China has 1.3 billions people it is willing to sacrifice while the US has 4 time less, half of it being obese, plus they must censor the pics of coffins coming back from war because dying americans is not acceptable.
- China host all the factories, and provide a huge chunks of primary resources to the US.
- China is capable of being self-sufficient rapidly, as they have the land and the capabilities of going back to manual labor.
- China army has millenias of terrifying history. It may very well be still here in 10000 years. The US has 500 years of history, total.
- The US never entered a war that would affect their borders. Up till then, they brought the fire far from home, giving a false sense of indestructibility to their population. China is your neighbor.
- Attacking China means WWIII, it's not sure humanity can even survive it.
It's not a guaranteed success but it would be sending a very strong signal. Better than nothing, perhaps
China could just as easily see as act of war or at very least, threatening their sovereignty. Maybe they sell their treasuries or restrict rare earths, or simply cut off supplies amongst their $400 Billion goods surplus with the US. I really don't think they'll allow themselves to be pushed around on this, out of general principle.
US should strengthen its support of Taiwan in retaliation.
Park a carrier strike group offshore, guarantee safe passage and refugee status to any Hong Kong resident wanting it, and provide transport to a host country.
It would take tremendous political will and resources. But this would be the non-violent, humanitarian and decisive way to intervene.
At which point do you think the first bullet of the war fires ?
Sure, it's not like anyone can or willing to do much about that, but the UK can play the Russian game (with some help from NATO), and provide help for the BN(O) people.
Shooting would be almost inevitable, but that at least establishes that there is a problem.
And why would Xi risk WW3, he and China has the most to lose, after all the future, with the current trajectory of the world looks to be theirs!
People assume that an intervention would be ships and helicopters showing up with people dancing in the streets like in some sort of movie.
Regime change does not look like that.
It looks Venezuela, Libya, Iran ...
And starts with a long information campaign.
Given these examples, Hong Kong would be much better off without such intervention.
I don’t think Venezuela, Libya, or Iran is a fair comparison to Hong Kong regarding the progress or prospect of liberation/democratization. For one, Hong Kongers have been used to elections for almost 30 years now (since 1991), have practiced free press and rule of law, and mostly free of corruption, so I think Taiwan may be a closer comparison in terms of how liberation/democratization of Hong Kong would become.
The recent frustration towards elections in Hong Kong (which is also one root cause of the on-going protests) is that the elections are far from representative or universal suffrage (half of the seats in the Legislative Council–the functional constituencies–have pretty limited representation, while the Chief Executive is elected by a pro-Beijing committee of 1200 out of a population of 7M+), and that some elected members of the Legislative Council who are pro-Hong Kong got outright disqualified. A liberation/democratization of Hong Kong means such issues are fixed, which is more like Taiwan than Venezuela, Libya, or Iran.
That's like saying Germany was a broken state after US intervened in WW2. I mean, it was literally broken up, but it didn't have an insane dictator committing mass scale human rights abuses anymore.
However, the simple fact that the US went in guns blazing and left before things got better (because otherwise it would have been too costly) doesn't invalidate all forms of intervention ever and forever.
It's no wonder such sensitive situations were mishandled. The theoretical magical chain of events that lead to sustained "freedom" was always there, on paper at least, but using the US examples we should conclude that its cost-benefit is waaay way lower than necessary to consider it a useful foreign policy tool.
Of course, an avid idealist might argue that this just means the US (or the world, or the West) just needs more experience to be able to better export "freedom"; and of course it is something we should never stop trying. And even today, even after the whole ISIL/ISIS campaign every place is better off now than it was before. (No Saddam, no Gaddafi, etc.)