magine what would happen if a British prime minister in London told the Scots they could not have their own parliament, national football team or fly the saltire. Or if the Welsh were told their language and poetry were banned and must no longer be taught in schools. The ensuing uproar would probably destroy the United Kingdom. Yet that's exactly what Xi Jinping, China's communist emperor, is telling the roughly 125 million members of the 55 ethnic and religious minorities who together comprise about 9% of the country's (majority Han Chinese) population.

Xi claims his sinicisation policy, embodied in a new "ethnic unity" law, will boost national identity and integration and, by expanding compulsory use of the Mandarin language, improve life and work prospects. Minorities should "hug tightly like pomegranate seeds" to build a strong, united China, he says. But Tibetans, Mongolians, Uyghurs and others who value their separate language and customs fiercely object to what they see as forced assimilation into a conformist, intolerant monoculture dominated by the Communist party's tunnel vision. For them, it's ethnic cleansing by another name.

The new law has been condemned, though not too loudly, by western governments and more vociferously by rights organisations such as Amnesty International. The death of Lobga Rangzen, a Free Tibet activist who set himself on fire in protest outside the UN in New York last month, briefly drew international attention. But such tragedies are not new. Wholesale repression in Tibet and brutal "re-education" camps in Xinjiang are but two longstanding aspects of Xi's intensifying nationwide war on dissent, difference and diversity. The law and its harsh penalties, which extend to Chinese citizens based abroad, adds legal weight to existing efforts to enforce his and the party's will (much the same thing these days) by creating a politically compliant, cowed population that dare not answer back.

A debilitating fatalism has taken hold in the west about China's behaviour, whether it involves persecution of minorities, evisceration of democracy in Hong Kong, vindictive prosecutions of pro-democracy activists such as the former Apple Daily publisher Jimmy Lai, or vicious suppression of independent Christian churches and "unapproved" religious groups. It's as though Xi's China is now seen as simply too big, too economically powerful and too militarily threatening to influence, let alone contain or control. When China's internal affairs are the issue, it's easier to steer clear. Awkwardly balancing trade against security, and culpably silent on human rights, Britain is far from alone among western democracies in failing to articulate a coherent China policy.

But Xi's increasingly aggressive, uncompromising policies are not confined to domestic matters. In power for nearly 15 years, Xi, 73, is growing ever more confident and abrasive in pursuit of his international legacy bucket list: establishing China as the dominant global power, relegating the US to second place and making his country the essential hub of a new multipolar security, economic and diplomatic order. His legacy list includes a takeover of Taiwan before he stands down, and unchallenged supremacy across the Asia-Pacific region. It all points one way: to Xi's coronation as the greatest Chinese leader since Mao Zedong.

With the 21st party congress approaching, 2027 could be a big year for Xi – and he is already getting his ducks in a row. His dictatorship looks unassailable. Top regime cadres, meeting this week in secretive conclave at the Beidaihe seaside resort in Hebei province, are expected to endorse his redoubled drive to enforce loyalty and discipline in the ranks – the official phrase is "sustained advancement of full and rigorous party self-governance". The Beidaihe gathering, and another meeting due in October, come amid almost Stalinist purges of senior political and military figures.

Signs of a more assertive international posture are everywhere. In recent months, China has stoked nationalist tensions with Japan and deliberately provoked the Philippines in disputed areas of the South China Sea. Its intimidation of Taiwan is unceasing. This week, Japan's latest defence white paper singled out China as presenting "an unprecedented strategic challenge" and warned the threat was moving closer to home. Admiral Samuel Paparo, head of US Pacific Command, fired a broadside at China's "coercive actions" in pursuit, he said, of regional dominance.

Xi is now actively challenging the US and the west on several other fronts. China is reportedly planning to provide Iran with up to 400 shoulder-fired missiles and new air defence systems. It is already materially assisting Russia's aggression against Ukraine. And an emboldened Beijing is deploying the sort of trade weapons beloved of Donald Trump. Last year it restricted exports of vital rare earth minerals, to Washington's great alarm. Only this week, China hit back at Trump's new tariffs by blocking sales of drone technology.

Xi's ever more bullish stance owes much to Trump's trashing of the post-1945 global order, disdain for international law and national sovereignty (as seen in Venezuela and Greenland), and strained relations with traditional allies such as Japan, South Korea and European Nato countries. Many now view the US as an unreliable partner. For China, Trump's shifting focus away from Asia and towards the western hemisphere is a golden opportunity to be exploited while it lasts. That may be one reason why Xi has taken the unusual step of agreeing to visit Trump in Washington next month, so soon after their May meeting in Beijing.

The main focus of the summit is expected to be artificial intelligence, amid US worries that it is in danger of losing the AI race. According to Chinese commentators, at least, Trump asked for the meeting after China agreed AI partnerships with 28 countries, mostly from the global south. AI is "probably the biggest thing anybody's ever seen", Trump said last month. "Whoever wins that race is probably going to win, period." By which he presumably means, win the 21st century's superpower crown.

The summit is likely to confirm the impression left by the last one: that Xi, outmanoeuvring Trump on Asia-Pacific security and Taiwan, accumulating geopolitical clout, and winning trade and tech wars, is well on the way to fulfilling his ambition of gaining the upper hand over the US. Yet in the end, Xi is still an unelected autocrat, a thug in a suit who relies on force to stay in power. He's failing to deliver economically and, as in India and elsewhere, young urban Chinese may not tolerate his old-school brand of suffocating authoritarianism indefinitely. Who knows? Perhaps his ethnic unity law may ultimately prove a step too far – as it certainly would in Britain.

All should be on their guard. Xi, no friend of the west, may be morphing into an outright enemy while all eyes are on Iran, Gaza and Ukraine. Symbolically if not yet physically, a dangerously unconstrained Xi is shutting the door on peaceful coexistence at home and abroad.

• Simon Tisdall is a Guardian foreign affairs commentator