Illustration: John Shakespeare Beijing sent a coast guard ship to disrupt a standard Indonesian operation to prevent illegal Chinese fishing. The Indonesians seized the Chinese fishing vessel in the Natuna Sea. But as they towed the confiscated boat to Indonesia, the Chinese coast guard rammed the fishing vessel, allowing it to escape. Jakarta called in China's diplomats to protest formally and the Indonesian maritime security chief, Arif Havas Oegroseno​, said the Beijing had created "a new ball game". Beijing, unrepentant, said that the incident occurred in "traditional Chinese fishing grounds". Indonesia's Oegroseno retorted that a claim of traditional fishing grounds is not recognised under the 1982 United Nations Convention on the Law of the Sea:

Donald Trump's comments are likely to alarm both China and US allies in Asia who fear China. Credit:Edgar Su "It's very fake, ambiguous, in terms of since when, since what year does it become historical, traditional?" Indonesia joined the lengthening list of Asian states angry and anxious about China's assertiveness. A senior member of the Indonesian Cabinet, Coordinating Minister for Politics, Legal and Security Affairs Luhut Pandjaitan​, said that his country would respond by sending more troops and better patrol boats to its naval base in the Natuna Sea. And on Monday, Japan officially fired up its latest radar station, part of Tokyo's stepped-up efforts to respond to China's assertiveness in the East China Sea.

Unfortunately, the US under Barack Obama spent more than a year in frozen immobility as China plunged into a frenetic program of base-building on disputed islands in the South China Sea. The islands are also claimed by the Philippines, Vietnam, Taiwan and others. The US finally stirred into action in the last few months. America, like Australia, makes no claims and takes no sides in the disputes, but urges all sides to avoid any destabilising steps. The US has, however, moved to reassure its regional allies that it stands firm in supporting them. Two weeks ago, for instance, it signed an agreement to rotate US forces through five Philippines military bases, a deal that the American ambassador to Manila described as "a pretty big deal". China's official newsagency, Xinhua, responded by accusing the US of "making the Asia-Pacific into a second Middle East".

Into this tense and difficult situation crashed the leading Republican candidate for the US presidency. Donald Trump told the New York Times that, as president, he would "perhaps" lay claim to one of the disputed islands of the South China Sea for the US. This is idiotic and potentially incendiary in one of the world's most flammable strategic tinder boxes. The US would be transformed instantaneously from being the guarantor of stability to being a great force for instability. It would be tantamount to an American declaration of hostilities against China, in return for nothing it actually wants. Simultaneously, it would insult the sovereign claims of US friends and allies who also claim the same islands.

And there was more. Trump said that he would start withdrawing US forces from the two biggest US bases in Asia, those in Japan and South Korea, unless those countries paid more towards the cost of the bases. "I would not do so happily, but I would be willing to do it," he said, without acknowledging that Tokyo already pays most of the costs of the American bases on its soil and South Korea more than a third. And if Japan, feeling exposed without reliable US backing, decided that it needed to go nuclear as a result? "I'm not sure that would be a bad thing for us," Trump said. The US was "basically protecting Japan" he said, and "at some point we cannot be the policemen of the world." "We're a country that doesn't have money," he said, striking one of his favourite themes.

"We were a rich country with a very strong military and tremendous capability in so many ways. We're not anymore. We have a military that's severely depleted. We have nuclear arsenals which are in very terrible shape. They don't even know if they work." The prospect of a nuclear Japan is deeply alarming for countries including China. Trump could be president. He's likely to be the Republican candidate and, according to the US betting markets, he has about a 40 per cent chance of winning the presidency. Hillary Clinton, the likely Democratic candidate, is rated a better chance at 60 per cent. Even so, it's unsettling for the Asia-Pacific, the Indo-Pacific and the wider world for one of the leading US candidates to speak so blithely, so erratically, so carelessly about ripping up some of the deepest security foundations of the modern world.

In Beijing, Trump's ideas play into the hands of military hawks. If a future US president could so upend the remaining structures for stability in the region, then the rational response is to prepare for a much more unstable situation. And that's a case for even more Chinese armaments and an even more assertive posture. And in the capitals of US friends and allies, Trump's musings will strengthen the hand of those arguing that the US is an increasingly unreliable ally. And that means these countries are more likely to accelerate their own arms build ups, or more likely to prepare to yield to China's demands. Either way, it's a recipe for destabilisation and danger. Even if he never sets foot in the Oval Office, Trump is a danger to the stability of Australia and the entire region.

Peter Hartcher is international editor.