Travellers to the city of Nanjing are often shown its memorial to the massacre committed there by the Japanese army in 1937, which claimed some 200,000 victims. Few go to the Zhanyuan Gardens, where a museum commemorates the Taiping rebellion, a 19th-century civil war in which 20 million Chinese people died.

The museum is housed in graceful buildings that look as if they belonged to the era of the Confucian literati. In fact, they were built in 1958 on the orders of Mao Zedong, who wanted to honour the Taiping as ‘a great peasant movement’.

They stand on the site of a palace occupied by one of the numerous rebel kings, razed to the ground when imperial forces of the Manchu Qing dynasty, backed by the major powers, retook Nanjing from the rebels in 1864.

Western statesmen of the time claimed that foreign aid to the dynasty was a necessary intervention to restore stability and protect commerce. Yet many in the West had sympathised with the rebels upon discovering that they were inspired by the Bible, only to be repelled by their cruelty and by the evident mania of their leaders.

Walking through the tranquil gardens and well-kept exhibits of weapons, coins and texts, one is transported back to a strange unearthly period in Chinese history when the masses, fired by a charismatic figure who said he was the younger brother of Jesus Christ, rose up against their overlords to establish the Taiping Tianguo or ‘Heavenly Kingdom of Great Peace’. At its peak the kingdom reigned over the wealthiest parts of China for a decade.

The Taiping Rebellion is the forgotten conflict that may have claimed more lives than any other civil war in history. It convulsed China and dealt a death blow to the pretensions of the old order. Karl Marx thought it the precursor to world revolution against the industrial system. It promised to bring justice, reform and Christianity; instead it brought terror, ruin, plague, famine and death on a biblical scale. Not surprisingly, many in China and the West would prefer to forget about it.

The result of intervention was that China staggered into the late 19th century under a weak dynasty, unreformed, its social structure shattered, heavily in debt to foreign powers and unable to wage war or make peace effectively. The inheritance of the uprising was seized upon by Mao and regularly inspires his successors to warn of the perfidy of foreign powers.

The Taiping Rebellion thus remains a lesson in the unforeseen consequences of engagement by outsiders in the internal affairs of a vast, complex, civilisational state.

The literature in English on the uprising is scant compared to the countless books written on the civil war and the Communist revolution of 20th-century China, which are easier to explain.

Now is a good time to read the best offerings on the Taiping period, because what to do about China is at the core of the American presidential contest. It consumes policy-making in Brussels, while nations in the so-called ‘Global South’ weigh up their interests between the new autocratic economies and the global trading and security systems set up by the West.

The sharpest contribution to the American debate has been by Matt Pottinger and Mike Gallagher in the pages of Foreign Affairs, which argues that the United States should not settle for managing China but should aim to win the new Cold War. The authors, a former deputy national security adviser in the Trump administration and the Republican former chair of the House Select Committee on the Chinese Communist Party, say that China ‘is pursuing a raft of global initiatives designed to disintegrate the West and usher in an antidemocratic order’.

They call for tougher diplomacy, more defence spending, strict trade policies and steps to sever China’s access to technology, all of which are the currency of bipartisan debate in Washington. But Pottinger and Gallagher go further than mainstream discourse by posing the big question – Lenin would admire this – ‘What would winning look like?’

Their answer is that imposing costs on President Xi Jinping ‘for his policy of fostering global chaos’ would force China to give up trying to win a ‘hot or cold conflict’ with the United States. Their provocative conclusion?: ‘And the Chinese people – from ruling elites to everyday citizens – would find inspiration to explore new models of development and governance that don’t rely on repression at home and compulsive hostility abroad.’

That sounds to some like a coded call for regime change. It has elicited a predictable chorus of disapproval from a swathe of the American policy establishment, which much prefers exploring nuance to taking decisions.

None the less, the authors are serious people – Pottinger is a fluent Mandarin speaker steeped in experience of the country – and do not call for anything so crude as interfering in Chinese politics, instead advocating stronger deterrence. Perhaps the issue of what might happen if an economic collapse or a lost war led to the fall of the dictatorship is too daunting to think about. At least somebody has now asked the question out loud.

It is, in fact, the same conundrum which vexed the great powers of the 19th century. Like the decaying Ottoman Empire, the realm of the Qing Emperors was in decline. Rotted by bribe-taking and superstition, China had failed to match the scientific progress and dynamic entrepreneurialism of the West. Its enigmatic Manchu rulers governed by ‘the mandate of heaven’ while employing a bureaucracy of Han Chinese officials to run the country. It was ripe for disruption. Yet when intervention came, it was not to change the regime but to preserve it.

The story of the Taiping rebellion is told with verve by Stephen Platt in his masterful history Autumn in the Heavenly Kingdom and in a vivid portrait of the Taiping leader, Hong Xiuquan, by the late Jonathan Spence, God’s Chinese Son. Platt calls the foreign engagement  ‘a reminder of just how fine the line is that separates humanitarian intervention from imperialism’.

There is also an insightful essay by the late Philip Kuhn in the Cambridge History of China, which makes the key point that ‘foreign contact itself had provided a new historical catalyst: an alien religion that generated a furious assault on China’s existing social structure and values’.

That is not lost on Xi Jinping, who governs as a modern emperor, employs his own court theologian of Sino-authoritarianism, and never ceases to remind his subjects of the insidious foreign plots which aim to sabotage the renaissance of their nation. One of his priorities is to ‘Sinicise’ all religions under state supervision. Pottinger and Gallagher cite him as saying that ‘the battle for mind control happens on a smokeless battlefield’.

Those who applaud Pottinger and Gallagher’s boldness in raising the question of what ‘victory’ over the Xi regime might look like can simultaneously feel uneasy at the idea that China might freely and peacefully find its way to a new system of governance after the dictatorship.

The Singaporean scholar-diplomat Kishore Mahbubani has written that a democratically elected successor to the Communist Party could be ‘far less rational’ in managing Chinese nationalism. He, too, should be taken seriously. So what does history tell us?

In Jonathan Spence’s book, the spark that lit the Taiping rebellion was struck in 1836 on a crowded street in Canton, the modern Guangzhou, when an aspiring candidate for the imperial examinations, Hong Huoxiu, took a tract handed out by a Chinese convert to passers-by. It was entitled Good Words for Exhorting the Age, a collection drawn from scripture. This was one of many translated works distributed by Protestant missionaries up and down the China coast.

Hong was a Hakka, whose ethnic group clustered from the central plains to the southern littoral. He failed the examinations and went on eking out his life as a teacher paid in scrip, food, lamp oil, salt and tea. The new doctrine appealed to his sense of injustice and redemption, in due course convincing him of its righteousness and – fatefully – persuading him that he was the saviour chosen to bring heaven down to the yellow earth.

By 1850 he had changed his name to Hong Xiuquan, incited a peasant uprising in the southern province of Guangxi, evangelised millions with his erratic version of Christian theology and turned their struggle into a campaign against the Manchus infused with racial hatred. The next year he proclaimed the Heavenly Kingdom and anointed himself as its Heavenly King. He conquered the imperial capital of Nanjing and seized huge territories of fertile, well-watered agricultural land.

Word of these events reached the British colony of Hong Kong when a Christian convert, Hong Rengan, a cousin of the Heavenly King, met a Swedish missionary named Theodore Hamberg and told him of a movement that sounded little short of miraculous. Enthusiasm spread among the devout, Hong Rengan was sent to work in Britain with the London Missionary Society. He later rose to high rank in the Heavenly Kingdom and, until his flight and death, continued to speak of his dreams of a Christian China of free trade, railways, newspapers and general progress.

The West had its own conflicts with the dynasty. Britain fought the Opium War of 1839-1842 to force open the door to trade. France, Japan, Russia and the United States all competed to profit from the opening. In 1857 the allies took Canton and in 1860 an Anglo-French expedition captured Beijing and burned the emperor’s summer palace to the ground. The court fled and the emperor expired a year later.

Throughout these years a dizzying sequence of battles and massacres unfolded across the south-east provinces. Powerless against foreign military technology in the north, the dynasty was able to wage old-fashioned war in the south. It appointed a pitiless general, Zeng Guofan, to lead the campaign. There were defeats and debacle, prompting Zeng to attempt suicide. Somehow, he and his imperial masters clung on.

What turned the tide was a change of heart in the West. Marx was not alone in seeing the Taiping as harbingers of change, writing that ‘the Chinese revolution will throw the spark into the overloaded mine of the present industrial system and cause the explosion of the long-prepared general crisis’. The Economist mused that it was ‘a social change or convulsion such as have of late afflicted Europe’, demonstrating that the Celestial Empire was already integrated into the global economy.

The Times raised the question of whether Britain should intervene, and on whose side, observing that the Taiping Rebellion was ‘in all respects the greatest revolution the world has yet seen’.

Early on, the rebels attracted lively sympathy among Christians. The Taiping spared Shanghai from conquest, assuring the foreign powers that thousands of their citizens were safe. Later, in the whirlwind of battle, British and French troops attacked Taiping forces near the city, but both sides refrained from a general conflict. Diplomatic and political contacts multiplied; indeed, the Taiping were compared by many to the Confederacy in the southern states of America and British policy was said to be neutral.

Platt’s work reminds us that the American civil war of 1861-65 was raging throughout the late stages of the Taiping rebellion – two world powers, both important markets for Britain, were engulfed in destabilising conflict. The Americans were distracted while the British and French secured gains in China and Russia grabbed the empire’s far eastern territories, an exaction that still rankles in Beijing today. But after 1860 the West was invested in the Qing Empire – depending on it to pay its sovereign debts, to conduct trade, to keep its pacts and to regulate law and order.

The subsequent defeat of the Taiping unfolds through blood-drenched pages in Platt’s narrative. Britain sent one of its most famous soldiers, General Charles Gordon, to rally the imperial forces. The American mercenary Frederick Townsend Ward led the ‘Ever Victorious Army’ to success before his own death in battle. But it was the patient Chinese commander Zeng Guofan who wore down the enemy, marching through sacked cities and burnt-out hamlets until he reduced Nanjing, slaughtered the population and executed the leaders, apart from the Heavenly King, who had died.

If the empire still clung to the tattered mandate of heaven, the Taiping had long lost support abroad. The Heavenly King was deranged, his junior monarchs warred among themselves, the supposedly enlightened administration of their realms fell into chaos and foreigners were appalled by Taiping massacres of captured Manchu civilians, which betrayed the racial, as Platt says, even genocidal, aspects of their ideology.

When the cities of the heavenly kingdom fell into a funereal calm, there was little time for introspection in the West as the world hurried on through the end of the 19th century and into the 20th, which brought a second civil war to China. The Taiping rebellion was one of many blows which hastened the decline of imperial rule and paved the way to Mao Zedong’s own utopian experiment, a consequence which we all live with today.

What, in hindsight, might the West have done differently? First of all, there are problems of definition. As Platt asks, was this a rebellion, a civil war, a national revolution or merely a descent into anarchy? In any case it aroused strong emotions among God-fearing people in the West, who knew little of the racial, religious and social tapestry that was Qing China. Spence cites a well-meaning clergyman who said the Chinese were ‘as intelligent and as wronged as the lamented Poles’.

There was never any possibility that western arms might have brought to power a liberal reforming government in China based on Christian values. The foreign powers were rivals, incapable of concerting common interests beyond the day to day scramble for advantage, while the Manchus bought time and the plentiful Chinese reformers of the late 19th century failed to gain political space.

The Japanese statesman Ito Hirobumi said in 1909 that British intervention was the most significant mistake the British ever made in China. By that he implied that preserving the throne merely prolonged an artificial state. Extreme Japanese imperialists believed that China was too big to be one country; indeed, the Communist Party’s obsession with unity stems from a deep-rooted fear of division at the hands of foreigners.

Hindsight is a wonderful thing, of course. It is worth recalling that admirals, diplomats, traders, missionaries and journalists all thought that Taiping China was acquiring the lineaments of a functioning state while it occupied the wealthiest, most populous regions of the country for more than a decade.

In Platt’s words, it was seen by some as ‘a competing government, a competing state, a competing vision of what China should be’.

Such delusions were dangerous. They led to a piecemeal approach when pragmatism and commerce ultimately decided the case in favour of propping up the dynasty. It was done without enthusiasm or concerted objectives or much of an overall strategy. Platt sums it up by saying that foreign intervention was critical but also ‘largely informal, often halfhearted, morally fraught, and in many ways effective purely by accident’

Those are terms which resonate with ominous familiarity today.