US President Donald Trump’s abrupt decision to pull all American troops out of Syria is yet another chilling reminder that those who believe in pledges and assurances made by the US do so at their peril.

While his generals and European allies may fret over the geopolitical implications of his capricious move, it is the US-backed Kurdish forces, fighting on America’s behalf against Islamic State in northeastern Syria, who will bear the brunt of the repercussions.

It is almost certain Turkey, which has long labelled them as terrorists inciting its Kurdish minority to secede, will carry out its threat to move in and crush them.

Deserted by the Americans who have been funding, training and arming them, the Kurds will pay for Trump’s perfidy with blood. He may have made good his campaign promise to pull out US troops but to the Kurds, he has just stabbed them in the back. And they say this openly, in so many words, to the world’s media.

South Korea, Japan and Taiwan must be watching this development – this breach of faith – with trepidation. So should other countries in the Asia-Pacific region, courted by the US in its thinly disguised attempt to contain China.

Seoul and Tokyo, especially, could not have forgotten that soon after Trump met North Korean leader Kim Jong-un in Singapore in June, he called off a long-scheduled military exercise between South Korean and US forces – just like that, without any prior notice to Seoul? Or that he has signalled more than once his aversion to keeping US troops in South Korea.

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No one can be sure now that he would not also, in a moment of impetuosity, announce that withdrawal via Twitter at 3am.

With America’s reliability as an ally brought into serious question, it is little wonder Seoul and Tokyo will want to hedge their bets. Hence South Korea’s quickened pace in reaching out to the North, and Tokyo’s signals to Beijing that it seeks a thaw in their frosty relations.

Meanwhile, Taiwanese are no doubt put on notice of the dangers that await them should they allow themselves to be used as pawns by the US in its bid for strategic dominance over China.

How the Trump administration ditched its friends in Syria is a wake-up call like no other.

Indeed when it comes to honouring its promises and assurances, the US has a fickle history – from failing to return islands in the South China Sea seized by Japan from China, as agreed at the Cairo Conference in November 1943, to leaving Hungarians to their fate when Soviet troops moved in to crush their 1956 uprising, which the Americans had encouraged.

Vietnam, which the US has wooed assiduously as a bulwark against Beijing’s ambitions in the South China Sea, should also remember vividly how Americans deserted their allies as the Indochina wars wound to a stop in 1975.

No doubt many who lived through those events will recall seeing television footage of the last US Marine helicopter evacuating Americans from the rooftop of their embassy in Saigon on April 29, 1975 amid pandemonium.

But an even more poignant and shameful debacle had taken place in Phnom Penh three weeks earlier.

Japan plays China's game in Cambodia. Hun Sen wins

In early April, the US, having prompted General Lon Nol in March 1970 to oust Prince Norodom Sihanouk in a coup, decided five years later to abandon Cambodia to the Khmer Rouge forces closing in on the capital, a “bug-out” as then US secretary of state Henry Kissinger called it.

The US ambassador to Cambodia at the time, John Gunther Dean, earlier pleaded with his superiors in Washington but to no avail. He and all Americans were ordered to evacuate on April 12, which he later described as one of the most tragic days in his life, the day “the US abandoned Cambodia and handed it to the butchers”.

“We had accepted responsibility for Cambodia and then walked out without fulfilling our promise,” he said during an interview years later. “That is the worst thing a country can do. And I cried because I knew what was going to happen.”

After the Khmer Rouge took Phnom Penh, it drove 2 million of its inhabitants into the countryside at gunpoint. In the end, nearly all of them died from executions, starvation or brutal torture.

But a more stinging indictment of the US action came from Prince Sirik Matak, then deputy prime minister of Cambodia. Ambassador Dean, out of honour and decency, had offered him a ride on the evacuating convoy and thereafter, asylum. Prime Minister Lon Nol had already fled to Hawaii. Prince Sirik Matak replied in writing: “Dear Excellency and friend, I thank you very sincerely for your letter and for your offer to transport me towards freedom. I cannot, alas, leave in such a cowardly fashion.

“As for you and in particular for your great country, I never believed for a moment that you would have this sentiment of abandoning a people which has chosen liberty. You have refused us your protection and we can do nothing about it. You leave us and it is my wish that you and your country will find happiness under the sky.

“But mark it well that, if I shall die here on the spot and in my country that I love, it is too bad because we are all born and must die one day. I have only committed the mistake of believing in you, the Americans!”

This reply, reportedly circulated and read shamefacedly in the corridors of power in Washington, is part of the historical record. Its author was later captured by Khmer Rouge soldiers and killed – some reports said he was shot in the stomach and left to die over three painful days, while another said he was beheaded.

It is highly doubtful Trump read the letter before he ordered the Syrian troop withdrawal. Or that he would care to.