Seeing the limits of American power exposed by its war against Iran, China has issued an unusually blunt warning that the two countries could end up in conflict over Taiwan.

For a peevish and fragile US president on a visit to Beijing, this is a humiliation.

It is not something Donald Trump will feel. It will be disastrous for the region where China is already engaged in another land grab.

Already puffed up by hordes of flower and flag-waving children, honour guards and meetings in China’s Great Hall of the People, it is unlikely that he’d be offended by the offensive because he doesn’t care about Taiwan, is irritated by past commitments to protect the island, and sees the entire region as part of China’s legitimate sphere of influence.

“You’re a great leader. Sometimes, people don’t like me saying it, but I say it anyway. There are those who say this may be the biggest summit ever,” said the US president.

Later briefings from Xi Jinping’s office said that the Chinese leader had warned his guest that Taiwan “was the most important issue they faced and if handled poorly could push the entire US-China relationship into an extremely dangerous situation and cause the countries to collide or even enter conflict”.

The White House did not mention Taiwan in its readout on the two-hour meeting between Xi and Trump.

A squabble while at the “biggest summit ever” would shift the coverage of Trump away from the smiles and diminish his place in the world as a Big Man.

On top of that, Trump has repeatedly said that the US is the natural leader of the western hemisphere, Russia of central Asia and China of most of the rest.

The US and China have a truce in their recent economic conflict when America suspended triple-digit tariffs amid China’s threat to cut the supply of rare earth minerals that the technology sector relies upon.

Since then, the Iran war has sent oil prices soaring. This has affected both nations, but China continues to import sanctioned Russian oil and hopes to renew its use of Iranian oil at discounts.

Trump is facing midterm elections for his Republican party, is plunging in the polls and still has not figured out how to get out of the Iran conflict, having achieved none of his aims for the war.

The US has maintained a policy of “strategic ambiguity” towards Taiwan since it separated from the mainland when the communists took over mainland China in 1949.

Washington maintains its influence as the biggest arms supplier for Taiwan and supports its right of self-defence while also ”recognising” the Communist Party of China’s “One China” policy, which demands the reunification of the two entities.

This leaves Trump room to manoeuvre, or to wriggle out of long-standing systems of support for Taiwan.

Implicit in the “stay out of our backyard” message from Xi is a wider demand. China’s backyard included the South China Sea, where China is rapidly and aggressively changing the geography in its favour.

To the horror of the Indonesians, 11 countries in the ASEAN block, Australia and Japan, China has been building man-made Islands in the sea to extend their territorial waters and dominate a choke point that controls about a third of all global trade.

Warships from around the world patrol the sea and “buzz” China’s artificial islands in demonstrations of the international right of passage and to undermine China’s territorial claims to more and more water around the tiny military atolls it has built between its south coast and Indonesia.

China insists that it has sovereignty over all or most of the region. It further demands that ASEAN countries, which trade most heavily through the South China Sea, agree to “resist disruptions”, which is code for allowing other nations like the US to participate in any arguments over the international laws that govern the seas.

China also wants ASEAN to sign a code of conduct that is not binding on China but ends military cooperation between its members and outside powers.

Last week, the Philippines agreed to set up the ASEAN Maritime Center in its Cebu province to monitor the South China Sea.

In warning America to stay out of Taiwan, which is not yet under assault, China is also telling a diminished Donald Trump to look away from its other ongoing land grab, of the sea.