North Korea's substantial nuclear arsenal and improving intercontinental ballistic missile (ICBM) capacity pose a growing threat to America's security. But we need not face an immediate crisis if we play our hand carefully.

Given the bluster emanating from Pyongyang and Bedminster, New Jersey, Americans can be forgiven for feeling anxious.

Shortly after adoption of new United Nations sanctions last weekend, North Korea threatened retaliation against the United States "thousands of times" over. Those sanctions were especially potent, closing loopholes and cutting off important funding for the North. August is also when the US and South Korea conduct major joint military exercises, which always set Pyongyang on edge. In August 2015, tensions escalated into cross-border artillery exchanges after two South Korean soldiers were wounded by landmines laid by North Korea. This juxtaposition of tough sanctions and military exercises has predictably heightened North Korea's threats.

We have long lived with successive Kims' belligerent and colourful rhetoric - as ambassador to the UN in the Obama administration, I came to expect it whenever we passed resolutions.

What is unprecedented and especially dangerous this time is the reaction of US President Donald Trump. Unscripted, the President said on Tuesday that if North Korea makes new threats against the US, "they will be met with fire and fury like the world has never seen".

These words risk tipping the Korean peninsula into war, if the North's leader, Mr Kim Jong Un, believes them and acts precipitously.

Either Mr Trump is issuing an empty threat of nuclear war, which will further erode American credibility and deterrence, or he actually intends war next time Mr Kim behaves provocatively. The first scenario is folly, but a US decision to start a pre-emptive war on the Korean peninsula in the absence of an imminent threat would be lunacy.

We carefully studied this contingency. "Preventive war" would result in hundreds of thousands, if not millions, of casualties. Metropolitan Seoul's 26 million people are only 56km from the border, within easy range of the North's missiles and artillery. About 23,000 US troops, plus their families, live between Seoul and the Demilitarised Zone; in total, at least 200,000 Americans reside in South Korea. Japan, and almost 40,000 US military personnel there, would also be in the crosshairs.

The risk to American territory cannot be discounted, nor the prospect of China being drawn into a direct conflict with the US. Then there would be the devastating impact of war on the global economy.

US National Security Adviser H.R. McMaster said last week that if North Korea "had nuclear weapons that can threaten the United States, it's intolerable from the President's perspective".

Surely, we must take every reasonable step to reduce and eliminate this threat. And surely there may be circumstances in which war is necessary, including an imminent or actual attack on our nation or our allies.

But war is not necessary to achieve prevention, despite what some in the Trump administration seem to have concluded. History shows that we can, if we must, tolerate nuclear weapons in North Korea - the same way we tolerated the far greater threat of thousands of Soviet nuclear weapons during the Cold War.

It will require being pragmatic.

First, though we can never legitimise North Korea as a nuclear power, we know it is highly unlikely to relinquish its sizeable arsenal because Mr Kim deems the weapons essential to his regime's survival. The North can now reportedly reach US territory with its ICBMs. The challenge is to ensure that it will never try.

By most assessments, Mr Kim is vicious and impetuous, but not irrational. Thus, while we quietly continue to refine our military options, we can rely on traditional deterrence by making crystal clear that any use of nuclear weapons against the US or its allies would result in annihilation of North Korea.

Defence Secretary James Mattis struck this tone on Wednesday. The same red line must apply to any proof that North Korea has transferred nuclear weapons to another state or non-state actor.

Second, to avoid blundering into a costly war, the US needs to immediately halt the reckless rhetoric. Mr John Kelly, Mr Trump's chief of staff, must assert control over the White House, including his boss, and curb the Trump surrogates whipping up Cuban missile crisis fears.

Third, we must enhance our anti-missile systems and other defences, and those of our allies, which need our reassurances more than ever.

Fourth, we must continue to raise the costs to North Korea of maintaining its nuclear programmes. Ratcheting up sanctions, obtaining unfettered UN authority to interdict suspect cargo going in or out of the North, increasing Pyongyang's political isolation and seeding information into the North that can increase regime fragility are all important elements of a pressure campaign.

Finally, we must begin a dialogue with China about additional efforts and contingencies on the peninsula, and revive diplomacy to test potential negotiated agreements that could verifiably limit or eliminate North Korea's arsenal.

Rational, steady American leadership can avoid a crisis and counter a growing North Korean threat. It is past time that the US started exercising its power responsibly.

NYTIMES

• The writer was the national security adviser from 2013 till this year and the United States ambassador to the United Nations from 2009 to 2013.