It would begin because the present approach of leaning on China to pressure North Korea will likely fail. Donald Trump will grow angry at public snickering at the emptiness of his threats.
At some point, US intelligence will see a North Korean missile prepared for a test launch - and it may then be very tempting for a deeply frustrated president to show his muscle. Foreign Affairs describes just such a scenario in an excellent new essay by Philip Gordon imagining how Mr Trump might drift into war by accident: "He could do nothing, but that would mean losing face and emboldening North Korea. Or he could destroy the test missile on its launch pad with a barrage of cruise missiles, blocking Pyongyang's path to a nuclear deterrent, enforcing his red line, and sending a clear message to the rest of the world." Alas, no one has ever made money betting on North Korean restraint, and the country might respond by firing artillery at Seoul, a metropolitan area of 25 million people.
The upshot of a war would be that North Korea's regime would be destroyed, but the country has the world's fourth-largest army (soldiers are drafted for up to 12 years) with 21,000 artillery pieces, many of them aimed at Seoul. It also has thousands of tonnes of chemical weapons, and missiles that can reach Tokyo.
General Gary Luck, a former commander of US forces in South Korea, estimates that a new Korean war could cause one million casualties and US$1 trillion (S$1.4 trillion) in damage.
Mr Kurt Campbell, a former assistant secretary of state for East Asia and now chairman of the Asia Group in Washington, warns: "I do not believe there is any plausible military action that does not bring with it a possibility of a catastrophic conflict."
US Defence Secretary Jim Mattis knows all this, and he and other grown-ups in the Trump administration would resist any call for a pre-emptive strike. Concern about the North Korean response is what prevented Mr Richard Nixon from a military strike in 1969 when the North shot down a US plane, killing all 31 Americans on board. And it's what has prevented US presidents since from striking North Korea as it has crossed one red line after another, from counterfeiting US$100 bills to expanding its nuclear programme.
Yet I'm worried because the existing policy inherited from Mr Barack Obama is running out of time, because all United States and South Korean policies towards North Korea have pretty much failed over the years, and because Mr Trump seems temperamentally inclined to fire missiles.
When US Vice-President Mike Pence says of North Korea, "The era of strategic patience is over", he has a point: Patience has failed. North Korea is the strangest place I've visited, but it has made progress as a military threat. When I started covering North Korea in the 1980s, it had zero nuclear weapons. It now reportedly has about 20 and is steadily churning out more.
Worse, North Korea is expected in the next few years to develop the capacity to attach a nuclear warhead to an intercontinental missile that could devastate Los Angeles. US "left of launch" cyber warfare may slow North Korean efforts, but the threat still looms.
If a military strike is unthinkable, and so is doing nothing, what about Mr Trump's plan of nudging China to apply pressure on North Korea?
It's worth trying, but I don't think it'll work, either. China's relations with North Korea aren't nearly as close as Americans think. One North Korean once introduced me to another by saying, "The Chinese government doesn't like Kristof", and then beaming, making clear this was a high compliment.
President Xi Jinping of China will probably amp up the pressure somewhat, and that's useful - North Korean missiles are built using some Chinese parts - but few expect North Korean leader Kim Jong Un to give up his nukes. In the 1990s, North Korea continued with its nuclear programme even as a famine claimed the lives of perhaps 10 per cent of the population, and it's hard to see more modest sanctions succeeding now.
"North Korea will never, ever give up its nuclear weapons," says Jieun Baek, author of a fascinating recent book, North Korea's Hidden Revolution. Sanctions will squeeze the regime, she says, but not deter it. Instead, she urges greater measures to undermine the regime's legitimacy at home by smuggling in information about it and the world (as some activists are already doing).
The only option left, I think, is to apply relentless pressure together with China, while pushing for a deal in which North Korea would verifiably freeze its nuclear and missile programmes without actually giving up its nukes, in exchange for sanctions relief. This is a lousy option, possibly unattainable, and it isn't a solution so much as a postponement of one. But all the alternatives are worse.
And if Mr Trump tries to accelerate the process with a pre-emptive military strike? Then heaven help us.