* British think tank RUSI believes that a US-North Korea war is "a real possibility."
* The war could be started by either side. It "will not be surgical or short," the think tank said, and there would be "scenes of carnage."
* Even without nuclear weapons, such a war could kill hundreds of thousands of people in a week, produce millions of refugees, and wreck the global economy.
* The UK should refuse to unconditionally support the US in this war, RUSI said.
An acclaimed British defence think tank has warned that war between North Korea and the United States is "now a real possibility" and that peaceful talks may no longer work to bring down tensions.
In a report Thursday, the Royal United Services Institute said a war could start in two main ways: Either Pyongyang could strike if it believes the US is planning a surprise attack, or Washington could strike if North Korea fires test missiles near Guam or California.
"If this war is launched, it will not be surgical or short," Malcolm Chalmers, RUSI's deputy director general, said in the paper.
What the war could look like
The US would most likely open a war with a large-scale attack, including airstrikes and cyberattacks, with a goal to disable or destroy all of North Korea's military infrastructure.
If the US decides to launch this as a surprise, preventive attack, it may tell only "a very select group of decision-makers," RUSI said. Congress and allies, such as Britain, may be informed only a few hours in advance.
North Korea's neighbouring countries, such as Japan and South Korea, may also be given limited warning if the US were to keep the attack under wraps. China and Russia may even find out about such an attack only as it takes place.
In turn, Pyongyang is likely to strike South Korea, with whom the US has been strongly allied since 1950, with conventional, chemical, or even nuclear weapons. Such a war would destroy US-South Korea relations and paralyse the global economy, RUSI said.
Tens of thousands of people would be killed within a week, even without nuclear weapons, RUSI said. And if nuclear weapons were deployed, hundreds of thousands would die. About 150,000 Americans and 8,000 Brits live in South Korea, the think tank estimated.
North and South Korea would "become scenes of carnage," with millions of refugees spilling into neighbouring countries. China, which shares an 880-mile border with North Korea, doesn't want this to happen, multiple experts have said.
'Too late to try to stop' North Korea's nuclear programme
Sir Simon Gass, the former political director of the UK's foreign office, echoed RUSI's warnings.
He said on Thursday, according to The Guardian: "In my judgment, it is too late to try to stop North Korea's nuclear capability. It is there and it exists and I see very little likelihood that circumstances would arise in which North Korea would be willing to negotiate away its nuclear capability.
"There is a further question about ICBMs," intercontinental ballistic missiles, "but in terms of nuclear capability, the toothpaste is out of the tube."
The UN has passed multiple rounds of sanctions designed to cripple the North Korean economy and persuade the country to remove its nuclear programme. Pyongyang has undertaken various missile and nuclear tests, indicating that these sanctions haven't worked.
North Korea also fired two missiles over Japan over the past month and has vowed to produce missiles that could reach the US island territory of Guam and the US continent.
"There is no easy military option that can destroy North Korean nuclear capabilities without starting a wider war," RUSI said.
Trump and Kim's provocations aren't helping
RUSI's report heavily cited US President Donald Trump's ongoing provocations against North Korean leader Kim Jong Un. Trump in September began referring to Kim as "Rocket Man," including in a speech to the UN General Assembly, and Kim has responded by calling Trump a "mentally deranged US dotard."
Trump also threatened that Kim and his foreign minister, Ri Yong Ho, "won't be around much longer." Ri said this was effectively a declaration of war.
Gass condemned these actions, saying any attempts to negotiate with North Korea peacefully "cannot be helped by name-calling and exchanges of ritual insults by the main two parties to the conflict.
What Britain should do
If the US attacked North Korea, Trump would "probably telephone 10 Downing Street within an hour" of the start of the attack to ask for support, RUSI said.
The think tank, however, recommended that the UK refuse to provide unconditional support to the US as it did in 2003 with Iraq. Though Britain could join the US in a war against North Korea, it doesn't have to under NATO rules.
Both Japan and South Korea have warned of more provocative acts from Pyongyang around mid-October.
RUSI said: "By repeatedly emphasising the massive consequences of preventive strikes against North Korea's nuclear programme, senior Pentagon leaders are therefore sending a clear message to Trump, and to the American people: if you do decide to go ahead with this, do not say we did not warn you."
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