Trump's tweet is of such epic irresponsibility that some nuclear policy experts felt physically sick

At the end of the first working day of 2018 Donald Trump gave Washington the equivalent of a cardiac arrest. After a day of tweeting provocations to Pakistan, the Iranians and the Palestinians, the President wrote a final tweet of such epic irresponsibility that some nuclear policy experts in this town felt physically sick.

Donald Trump is the commander in chief of the US military. He has extraordinarily broad and largely unchecked power to launch a nuclear strike. So the one thing he should never do is joke about nuclear weapons or play games on social media about US nuclear strategy, right? That's especially true when it comes to dealing with the world's highest-stakes confrontation - the global effort to contain and ultimately roll-back North Korea's nuclear weapons and missile programmes.

And yet last night that is exactly what Trump did, needling Kim Jung-Un in a taunt worthy of a playground tantrum.

So American diplomacy has come down to this: The President of the United States declaring that his button is bigger (and more reliable) than the one controlled by "Little Rocket Man."

How humiliating for American diplomats and military officials.

Few saw the funny side of the tweet. Millions of lives depend on a diplomatic solution to the stand-off on the Korean Peninsula.

Former Obama administration officials are now openly declaring the President to be mentally unwell.

It had seemed - for a brief moment - that at the end of 2017, with a major legislative achievement under his belt (the tax reform bill) and with the economy and stock market booming, that Trump might consolidate his presidency. Perhaps he would avoid the puerile asides that characterized his first year in power and stop undercutting his own diplomats and policies.Just hours into the New Year that now seems hopelessly naïve.

I don't know how 2018 will end for this President or for this country. But I certainly don't like how it's begun.