President Donald Trump said the nuclear threat from North Korea was over despite having completed little to no work on actual denuclearization.

The US was headed to war with North Korea over their long range missile tests, and Trump getting North Korea to halt the tests stopped it, an Obama admin official told Business Insider

Trump's joint statement with North Korea paid lip service to denuclearization, which the US's Asian allies wanted, but the real success of the meeting may have been getting North Korea to freeze testing.

South Korean media now reports North Korea is getting ready to destroy an ICBM engine testing facility.

President Donald Trump left the Singapore summit with North Korean leader Kim Jong Un and declared the nuclear threat from Pyongyang over without even getting close to a concrete denuclearization process from North Korea.

But in doing so, Trump may have been heeding a warning from former President Barack Obama and stopping a possibly nuclear war with North Korea in the process.

"Before taking office people were assuming that we were going to War with North Korea. President Obama said that North Korea was our biggest and most dangerous problem. No longer - sleep well tonight!" Trump tweeted.

In November 2016 when President-elect Trump visited Obama in the White House, news outlets widely reported that Obama told him that North Korea would be his biggest threat, and Trump seems to confirm that here.

But the threat from North Korea wasn't its simple possession of nuclear weapons. North Korea first demonstrated nuclear capability in 2006, and had nukes throughout Obama's entire presidency, but Obama responded only with "strategic patience."

Instead, according to former US ambassador to Turkey, James Jeffrey, who worked for Obama, the warning centered around North Korea getting missiles that could strike the US, something US intelligence officials estimated would happen during Trump's term.

"A nuclear strike capability against the US changes the entire strategic equation in a way that just having nukes that can be exploded in South Korea and Japan does not," Jeffrey told Business Insider.

"It decouples the US deterrence and retaliation capability against any North Korean attack."

Basically, if North Korea has missiles that can hit the US, then it can ask Washington a terrifying question: Will you trade Seattle for Seoul?

The US has, for years, said its alliance with South Korea and Japan are "ironclad," but according to Jeffrey, that's diplomatic speak that masks a dark truth everyone already knows: The US would not take a nuclear attack from North Korea on the chin to save an allied city.

The US planned to attack North Korea if their tests continued

North Korean leader Kim Jong Un watches the launch of a Hwasong-12 missile in this undated photo released by North Korea's KCNA Thomson Reuters

So, as former National Security Adviser H.R. McMaster posited, having an ICBM would allow North Korea to attack South Korea without the US stepping in.

An ICBM "changes any attempt by North Korea to reunify the peninsula," said Jeffrey. Kim "can undertake a military or political pressure campaign on South Korea and America’s hands will be tied because we don’t want to risk losing Seattle for Seoul," said Jeffrey. "Thats what Trump was reacting to."

Trump was "basically told [by Obama] if North Korea continued their tests, and they need more tests to have a survivable weapon, that would we would strike. Probably a limited strike," said Jeffrey.

Although North Korea managed to display it had missiles that could reach the US, it didn't prove that it could mount a nuclear weapon on the missiles, that the missiles could hit an intended target, or that the warheads could survive reentering the earth's atmosphere at many times the speed of sound.

The only way North Korea could really prove it had real ICBMs would be to shoot a real, armed one and detonate it over the Pacific, or show off their accuracy by shooting missiles just short of some US territory. Once Trump took office and North Korea's missile system ramped up, they threatened repeatedly to do both of those tests.

According to Jeffrey, either one would have led to war.

Frank Aum, the Pentagon's senior advisor for North Korea under Obama, confirmed to Business Insider that there was a "general understanding that a red line would be an atmospheric nuclear test over the ocean or an [intermediate-range ballistic missile] test that lands in the vicinity of Guam."

Denuclearization a red herring, but Trump caught the bigger fish

President Donald Trump answers questions during a press conference following his historic meeting with North Korean leader Kim Jong-un June 12, 2018 in Singapore. Trump described his meeting with Kim as 'better than anyone could have expected.' Win McNamee/Getty Images

With what Trump has done, he can claim they’re on the road to denuclearization, which would be good if we get it," said Jeffrey. But Trump's real victory, the one that eliminated North Korea's real nuclear threat towards the US, was freezing their move towards ICBMs, he said.

Denuclearization was "not really what they were out to do," said Jeffrey, who said Trump operated under the guise of denuclearization to reassure South Korea and Japan, who remain under threat of North Korean nuclear attacks even without ICBMs.

Instead, Jeffrey said Trump went in with the narrow goal of getting North Korea to stop ICBM and nuclear weapon testing, and he got it. This explains why Trump settled for the weak joint statement that provided no concrete language on removing nuclear weapons.

"My feeling is that we’re in a process that is good, the process of psychological, military pressure, and economic sanctions has dealt pretty damn effectively with the problem Obama gave Trump," said Jeffrey.

What was the one North Korea concession Trump talked up after the Singapore summit? The impending closure of a missile engine testing site, which on Thursday South Korean media identified as a testing site for ICBMs — perhaps Trump's real goal in all of this.