The world was genuinely shocked when Donald Trump vowed to retaliate against North Korea "with fire and fury like the world has never seen" if it continued to issue threats to attack the United States.
But the world should not have been surprised. Only last weekend Mr Trump's national security adviser, General H R McMaster, told an interviewer that the president was "not going to tolerate" North Korea being able to threaten the US with a nuclear missile.
What was alarming was the change in tone. For decades US presidents, when discussing international relations, have followed Teddy Roosevelt’s maxim: “Speak softly and carry a big stick: you will go far.” With his warning of nuclear war, he has upended the calculus endorsed by much of the military top brass that the consequences of using nuclear weapons would be too catastrophic to contemplate.
What is certain is that in terms of military strike power, Mr Trump was not exaggerating. The first US nuclear weapon, used on Hiroshima in 1945, had a yield of around 15 kilotons of TNT. The second, dropped on Nagasaki, was slightly larger at 20 kilotons. Today's US nuclear bombs are more accurately targeted and far more destructive, with a yield of up to 340 kilotons, capable of destroying a whole city and its outskirts, and spreading fallout from North Korea into China.
Mr Trump is not the first US president to contemplate using military force to destroy the North Korean nuclear programme. In 1993, Bill Clinton, then US president, said that if North Korea used nuclear weapons, "it would be the end of their country".
The Clinton administration considered bombing Pyongyang's nuclear installations, but decided that the retaliation against the South Korean capital Seoul - only 55 km from the North Korean border – would lead to thousands of casualties. Instead Mr Clinton opted to open talks on freezing the nuclear programme.
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So what has changed in 25 years? North Korea is now estimated to have an arsenal of 60 nuclear warheads. It has a fast-developing missile programme apparently capable of striking as far as Los Angeles. US intelligence believes that North Korea is now able to miniaturise a bomb to put on an inter-continental ballistic missile. Such a weapon system could be ready for use next year.
Mr Trump has clearly decided that his predecessors were weak. The most generous assessment is that such threats will galvanise China into putting pressure on its ally to freeze the nuclear programme. The president has long believed that China is capable of doing this, and owes it to the US for, as he sees it, having got rich on one-sided trade with the US.
There is little chance that China will warm to this approach. Rather, it is likely to be seen as a sign of confusion in Washington. Only last Saturday the Trump administration scored a rare diplomatic victory when the United Nations stepped up sanctions against North Korea, with both China and Russia voting in favour. China promised to implement the sanctions, despite the cost to its own economy.
Chinese consent was won over by a diplomatic effort by the US secretary of state, Rex Tillerson, who declared that the US was not seeking options beyond sanctions - understood to be military ones - against North Korea. It did not desire regime change, nor was it looking for an excuse to "send our military north of the 38th parallel" that marks the border between the two Koreas. This is exactly what Beijing wanted to hear. In a hierarchy of threats as seen by the Chinese, the presence of US forces taking over North Korea and appearing on their border is more significant than North Korea possessing nuclear weapons. In fact, peace and stability would be greatly enhanced if the American army and navy quit the region and stayed in their own continent.
Not surprisingly, a Chinese expert dismissed Mr Trump’s nuclear threats as “irresponsible”. Even before Mr Trump’s statement, Chinese media had suggested that Beijing’s cooperation with Washington on the nuclear file depended on not using the US military as a threat.
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Decisive action from China to rein in its ally – they are bound by a mutual defence treaty though its status is somewhat uncertain – is unlikely. The consensus of experts on North Korea is that Kim Jong-un cannot afford to step back after he has put nuclear missiles at the centre of his defence against the Americans. Barring a Chinese-engineered coup, it seems likely that the nuclear and missile programmes will continue, even if the cost pushes the population into starvation.
Andrei Lankov, a veteran North Korea scholar, says that Pyongyang will be interested in diplomacy only once it has a proven ability to strike at the US. The regime will achieve that goal in a couple of years, and then it might be ready to talk of a nuclear and missile freeze, an offer which the US should accept. Despite the overheated rhetoric, cool heads will prevail and there will be no war.
All very logical. But this scenario ignores politics. What of America's credibility, which Mr Trump has promised to uphold, unlike Barack Obama who failed to enforce his "red line" against the Assad regime's use of chemical weapons in Syria? In January, Mr Trump tweeted of North Korea's boast that it was developing a nuclear weapon capable of reaching the US: "It won't happen!"
How it is not going to happen remains as unclear now as it was in January. Mr Trump has upped his rhetoric almost to the level of North Korea's, where threats to destroy Seoul or Washington in a "sea of fire" are a staple of propaganda. He now appears on the world stage as a dangerous, unpredictable actor, the opposite of the hyper-rational Obama. But is he so unpredictable that he would he launch a pre-emptive war of such cataclysmic consequence that it would make the Iraq fiasco look like a sideshow?
