Image copyright University of Maryland Image caption Ms Yang said the air in the US was "sweet and fresh"

Half a century ago millions of Chairman Mao's Red Guards gathered in rallies in Tiananmen Square to chant slogans and wave their red books of his quotations in a show of loyalty to the ideas of the "Great Helmsman".

The 21st Century successors to the Red Guards are not a physical presence. After the chaos of the Cultural Revolution and the tragedy of the Beijing massacre in 1989, young people are not allowed to demonstrate in China.

But some now hound their enemies online. The underlying rage is reminiscent. The instinct for intimidation is the same. Despite all its strengths and all its engagement with the world, China is once again prey to political groupthink and fear.

Media playback is unsupported on your device Media caption What did Maryland university students think of the "fresh air" speech

The latest trigger is a speech by a Chinese student at an American campus. On 21 May, at an official event, Shuping Yang praised the fresh air and freedom of speech she had found at the University of Maryland.

The video clip of her speech quickly went viral and triggered an outpouring of anger from fellow Chinese students in the US and critics at home. Shuping Yang swiftly apologised, asked forgiveness and said she had no intention to belittle her country. But that was not enough to stop the flood of "I am proud of China" posts accusing her of lies and deception, or the online "human flesh searches" to dig up incriminating information about her and her family.

Of course there are good reasons to be proud of China and every Chinese citizen is entitled to list them. In the past week alone, China has sent a submersible deep into the Mariana Trench and its world champion go player Ke Jie took on the AlphaGo AI computer programme and almost beat it. Every day ordinary Chinese people display the energy, talent and hard work of which their fellow citizens should be proud.

Image copyright Getty Images Image caption The Red Guard was Chairman Mao's ideological youth movement to "purify" the Communist Party

But being proud of China does not mean denying another Chinese citizen the right to an opinion. In fact, Shuping Yang herself said she too was proud in her message of apology.

The irony is that the very backlash against her has only served to make her point about the want of freedom of speech in her homeland. It has also highlighted a conflict between a commitment to free speech in Western countries that host large communities of Chinese students and the growing determination of the Chinese government and some of its citizens that free speech should be limited when it comes to talking about China, even beyond Chinese borders.

Freedom of speech is any society's feedback loop. It means precisely the freedom to say what is different or what may even offend. Of course, different societies have a different view on how much of this is appropriate. But if China's freedom of speech goes no further than parroting the leader and attacking those who dare to speak from a different script, then its spirit is lurking in the shadow of the Mao era.

Media playback is unsupported on your device Media caption Ms Yang said the air in the US was "sweet and fresh"

Which brings us to Chairman Xi and his style of leadership. In English Xi Jinping is usually referred to as President Xi. But his power comes from being leader of the Communist Party and since taking up that role five years ago, he has collapsed the distinction between party and government and dramatically shrunk the space for freedom of speech.

All public debate, whether in the media, academia, the legal profession or online, is a shadow of what it was in 2012. It is now off-limits to discuss universal values or liberal democracy. Instead China must loudly unite around the leadership of the Communist Party and "tell China's story confidently".

Confidence is understandable. In Xi Jinping's first five-year term, China has become the world's second-largest economy and an increasingly powerful military power. But when Chairman Xi urges journalists, think-tanks and diplomats to "tell China's story confidently" he does not mean tell it how you like and with your own nuance. Students abroad are a particularly important voice in this chorus. It is stated Chinese government policy to "assemble the broad numbers of students abroad as a positive patriotic energy".

And so when the University of California San Diego announced that it would host a speech by Tibet's spiritual leader the Dalai Lama next month, the local Chinese Students and Scholars Association consulted with diplomats and threatened "tough measures to resolutely resist the school's unreasonable behaviour". At Durham University in the UK, the Chinese Students and Scholars Association, again with the support of the Chinese embassy, attempted to bar from a debate a critic of China's religious policies and human rights record.

Image copyright AFP Image caption The Dalai Lama frequently gives speeches at universities around the world

This week's mobilisation against Shuping Yang, complete with commentaries in leading state media, is part of this drive for "positive patriotic energy".

All of which causes some bafflement on the campuses concerned. Students from countries with a tradition of free speech may feel irritation with someone who criticises their homeland in a public speech, but their instinct is usually to shrug it off or make a joke. Likewise when Chinese state media deploy students from Western countries praising China and its policies, such individuals do not become hate figures for outraged student associations or national newspapers.

Image copyright Getty Images Image caption Years after the real events, China has Cultural Revolution-themed restaurants

That's because liberal societies take differences of opinion for granted. In the US, in Europe and in Australia, citizens regularly excoriate their own governments and praise other countries in the media, and on satirical TV and radio shows. They also mount protests against their leaders.

It is vital to Beijing that these habits should not rub off. So in Chairman Xi's era the numbers of Chinese students studying abroad is going up but their tolerance of diverging views on China is going down.

In one respect, this is puzzling. At great expense, young Chinese have chosen to move from the confines of China's tightly-controlled education system to the "fresh air" of campuses which cherish tolerance and which offer all the tools to explore a range of different narratives of their own place in the world through reading and debate. But it is not so puzzling if you factor in these students' prior ideological education, the pressure on them to perform academically, and the ever-present and watchful eye of the Chinese state.

Image copyright @Admjeinsbt Image caption Ms Yang's apology on China's Weibo microblogging service

Tension is likely to grow between the liberal values of Western campuses and the "positive patriotic energy" of the growing numbers of Chinese students on these campuses. But the very strength of the reaction to Shuping Yang's freedom speech ensures that her words will continue to echo.

After all, it's not just Western culture which honours a loyal opposition. It is firmly entrenched in the historical memory of China too. Respect resonates down through the centuries for officials and soldiers in the imperial and the more recent Communist era who braved banishment or death for daring to speak truth to power.

So be #proudofChina by all means, but don't go back to the frenzy of the Red Guard era, and remember that in all great civilisations, the patriots whose memories endure are often those who love their country enough to point out its flaws.