It's the apocalyptic scenario that's sure to keep war planners up at night.

However, if North Korea followed through on its threat to fire an intercontinental missile towards the United States or her allies, one thing is certain — it would be ready to react within minutes.

North Korea sparked global alarm yesterday after testing its third ICBM following a 74-day pause in testing.

State media outlet KCNA said the test-fire of the intercontinental ballistic rocket Hwasong-15 was more sophisticated than any previously tested with a "super-large heavy warhead which is capable of striking the whole mainland of the US".

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Pyongyang said the missile reached an altitude of 4475km and splashed down 950km from its launch site, sparking a furious response from the US which warned Pyongyang "will be utterly destroyed" if war does break out.

Washington's ambassador to the United Nations, Nikki Haley, called on the international community to cut all diplomatic and trade ties with North Korea — including Chinese oil shipments to Pyongyang, following the provocative launch.

At least one Western expert said the missile's lofted trajectory suggested an actual range of 13,000km which is enough to hit every major US city.

David Wright, an arms control expert and co-director at the Union of Concerned Scientists, said the flight parameters showed this test pointed to a missile with "more than enough range to reach Washington DC, and in fact any part of the continental United States."

Inside the War Room

Former nuclear launch officer Dr Bruce Blair, who co-founded international nonpartisan group Global Zero, painted a minute-by-minute picture of how a North Korea missile attack could play out.

The nuclear security expert and researcher said a launch would be quickly detected and the early stage of the missile defence and nuclear retaliation protocols would be initiated.
"If North Korea launches a missile aimed at US or allied territory, the fiery hot plume of the booster would be detected within one minute by US satellites equipped with heat detectors," Dr Blair said.

Missile defences in Alaska and California and those in South Korean and Japanese territory as well as US (Aegis) destroyers would also be quickly notified.

At the same time an emergency conference involving the head of Strategic Command (near Omaha), the President and his top advisers would be underway, Dr Blair said.

"Within a few minutes, two key ground radar sites in Alaska (one at the end of the Aleutian chain and one in Clear, Alaska) would detect and analyse the missile path, providing further cuing information to the missile defence units which would begin to prepare to launch their interceptors designed to hit the missile in the middle of its trajectory (the late 'up' phase and the early 'down' phase) and the terminal re-entry phase in some cases," he said.

However, Dr Blair points out this isn't a guaranteed success with US missile interceptors having only around a 25 per cent success rate of having a head-on collision with the attacking missile. Some experts warn this rate is even lower than that.

Kim Jong-un signs an order document of a test-fire of the intercontinental ballistic rocket Hwasong-15.
Kim Jong-un signs an order document of a test-fire of the intercontinental ballistic rocket Hwasong-15.

The path of the missile would be confirmed within minutes.

"If it threatens North America, the protocol for nuclear or non-nuclear retaliation would continue and a presidential decision on retaliation rendered within a few additional minutes," Dr Blair said.

In the meantime, the entire US military would be on alert and their readiness raised two notches to a level known as Defense Condition Two, something Dr Blair points out has been reached during the 1962 Cuban missile crisis.

"On receipt of this notice, US forces would begin their preparations for retaliation and wait for the launch order," he said.

During this time, two other emergency plans would be underway, one of which is the Continuity of Government which ensures the safe evacuation of the President and other key government leaders, last implemented during the September 11 attacks in 2001.

If the President is at the White House, he would have only minutes to race from his blast-proof emergency operations centre under the east wing to his helicopter on the south lawn.

From here he would fly to a rendezvous base outside of Washington where he would board his Doomsday Plane, a militarised Boeing 747 possessing all the launch codes and the communications gear needed to send orders directly to the submarines, underground missile crews and bomber crews, Dr Blair said.

Homeland security would then initiate civil defence plans and the US population would be warned to take shelter while the President would try to address the nation via an emergency broadcast network.

If a single or small nuclear strike did occur first responders would be on the front lines, but the result would not be pretty.

"The civil defence effort would resemble a cross between Hurricane Katrina and Japan's Fukushima nuclear power plan disaster," Dr Blair said.

"First responders would be on the front lines. Nuclear radiation and contamination would greatly complicate the rescue and recovery effort, and in all likelihood would overwhelm the few hospital units equipped to handle radiation sickness."

'Totally destroyed'

Washington urged tough action as emergency talks on the North's latest provocation opened in the UN Security Council which follows US President Donald Trump calling Kim Jong-un a "sick puppy" and threatening "major" new sanctions.

In an angry response during a UN Security Council meeting, Ms Haley told her fellow envoys the only way to stop North Korea was for the global community to cut all ties with the regime.

"We call on all nations to cut off all ties with North Korea," she told the meeting.

Ms Haley said Mr Trump had called Chinese President Xi Jinping and urged him to "cut off the oil from North Korea".

"That would be a pivotal step in the world's effort to stop this international pariah," she said, issuing a stern warning to Mr Kim.

"If war comes, make no mistake: The North Korean regime will be utterly destroyed."

Effective launch

Pyongyang has yet to prove its mastery of the re-entry technology required to bring a warhead back through the Earth's atmosphere.

However, experts believe it is at least on the threshold of developing a working intercontinental nuclear strike capability.

Military think-tank Stratfor analyst Rodger Baker warned the middle of the night launch and the speed at which it took place represented enormous challenges for US missile defence.

"By testing late at night, and potentially even being able to move the missile out of its facility and launch it quickly — which is what appears happened — it was a way to demonstrate they have the ability to launch with the minimal ability of the US or South Korea to strike it on the pad before it got off the ground," he told CNBC News.

This means the window of opportunity for attack aircraft to scramble and get into place is much shorter.