"Our military is ready if necessary," Mr Trump said Thursday at the White House, before continuing to say that the summit cancellation is a "great setback for the world" and bragging about "greatly enhanced" military power the the US and its allies have.
"Likewise, I have spoken to South Korea and Japan and they are not only ready should foolish or reckless acts be taken by North Korea, but they are willing ot shoulder much of the cost of any financial burden — any of the costs associated by the United States in operations, if such an unfortunate situation is forced upon us," he continued.
Mr Trump also expressed hope for North Korea's future.
"Hopefully positive things will be taking place with respect to the future of North Korea," Mr Trump said. "But if they don't, we are more ready than we have ever been before".
"North Korea has the opportunity to end decades of poverty and oppression by following the path of denuclearisation and joining the community of nations, and I hope that Kim Jong-un will ultimately do what is right not only for himself but perhaps, most importantly, what's right for his people who are suffering greatly and needlessly," Mr Trump said.
Mr Trump's decision to withdraw from the summit has left American allies and politicians with the tough task of determining how the president would like to proceed regarding North Korean nuclearisation and missile testing. It also would appear to greatly reduce the chance that the Trump administration will be able to reach a peaceful agreement with Pyongyang that would lead to North Korea's abandonment of its nuclear and missile programmes.
It is not clear exactly how much input the Trump administration may have sought from South Korea or Japan before announcing the withdrawal from the summit.
South Korea presidential spokesperson Kim Eui-kyeom said in a statement that the country's officials are actively seeking to determine what the announcement means.
"We are attempting to make sense of what, precisely, President Trump means," he said.
US Secretary of State Mike Pompeo, who is testifying to Congress Thursday, refused to comment on whether Mr Trump's letter had blind-sided South Korean officials. Mr Pompeo had met with Kim Jong-un twice.
A statement from the Pentagon following Mr Trump's letter — which cited "tremendous anger and open hostility displayed" by North Korean officials as the reason for withdrawal — said that the US will continue its "maximum pressure campaign" against North Korea.
Tom Collina, who has worked with the Obama, Bush, and Clinton administrations on nuclear non-proliferation, told The Independent that Mr Trump's decision to pull out of the summit makes little sense, and that his sabre rattling does not send much of a message to the North Korean regime that they don't already know.
"It's kind of mindless boasting. Everyone knows the US has a large nuclear arsenal and a large military force," Mr Collina, the director of policy at the Ploughshares Fund, a foundation that supports initiatives to prevent the spread of nuclear weapons, said.
"The North Koreans know this ... it does not need reminding. For Trump, it just seems like he's boasting to his base, trying to make a point. This is typical Trump boasting fire and fury: My button is bigger than your button. None of it matters because North Korea has a nuclear deterrent. The US is not going to launch an attack on North Korea so long as North Korea has the option to rain down bombs on South Korea or Japan."
Ned Price, who served in the Obama administration as special assistant to the president and as a National Security Council spokesperson, told The Independent that Mr Trump's decision only serves to isolate the US from its allies through unpredictability, and that it is hard to tell how North Korea will respond — or how that response will impact any potential future negotiations.
"This always had to be something that we were in lock-step with our regional allies, primarily South Korea, but also China and Japan," Mr Price said. "This, I think, will be a reminder to all of those countries that Donald Trumps America can't be trusted. Even when we do the right things, and make the right moves one day, we may pull he rug from under them on the next."
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