US President Donald Trump cannot have his cake and eat it. He wants China to rein in what he characterizes as the growing threat of North Korea’s nuclear arms program. Yet at virtually the same time that he was telephoning Xi Jinping to ask him to do something about Pyongyang, Trump sent a US destroyer to sail close to Triton Island one of the disputed Paracel archipelagoes on which China has built a large military base.

Coming on top of the sale of $1.4 billion-worth of weapons to Taiwan and his administration’s criticism of human rights in China, it is hard to see how Xi would have been particularly welcoming when he took Trump’s call.

America is pushing back against Chinese military assertiveness in its own backyard. Though Washington does not like to lose its post-war hegemony, in its way Beijing’s policy is not dissimilar to the US Monroe doctrine. Way back in 1823, President James Monroe told European powers that any interference in Central and South America, where colonies were fighting for their independence from Spain and Portugal, would be viewed as “the manifestation of an unfriendly disposition toward the United States”.

North Korea is clearly a dangerous threat to two key US allies, South Korea and Japan. President Obama and his Secretary of State Hillary Clinton sought to convince Beijing that Pyongyang was also a potent threat to China itself. This was always a false proposition and it is hard to see how anyone in Washington could really believe it. North Korea’s economic and indeed financial survival depends almost entirely on Beijing’s goodwill. Without flows of Chinese oil and goods, including the many luxuries enjoyed by supreme leader Kim Jong-un and his governing military caste, this brutal police state would collapse.

Were that to happen the case for reunification would be pushed hard, not least by Washington. A single Korea in which the prosperous south of the country would pour capital and expertise into the north, might be a boon for the great majority of Koreans in the north, but it would also bring a close American ally right up to the Chinese border. What possible gain could Beijing achieve here?

Trump’s pressure that China stop Pyongyang’s increasingly aggressive nuclear posturing is therefore in vain. Even without what Beijing regards as his provocative despatch of a warship to sail close to Triton Island and his selling of sophisticated weaponry to Taiwan, President Xi had no good reason to fall in with Trump’s wishes.

North Korea is China’s very effective regional Rottweiler. Beijing is clearly confident that the economic collar and chain around Pyongyang’s neck will hold. Meanwhile the fearsome barking from Kim Jong-un is a convenient disturbance for Washington and its regional allies.

Hours after his victory, Trump took a call of congratulations from Taiwan. This caused fury in Beijing. It was seen as a demonstration of his determination to challenge China, not least over perceived unfair trading practices and its militarization of disputed reefs in the South China Sea. He set out his stall and it was one of confrontation — of putting America first. In such circumstances it is strange that Trump felt he could put in an urgent call to President Xi asking him to do something about North Korea. He really should have saved his dime.