Kiwi expert: How conflict between the US and North Korea would unfold

What would war with North Korea look like?

Waves of bombers, special forces secretly dropped inside the closed, insular nation and a massed lightning strike across the border - these are scenarios considered by New Zealand academic Dr Colin Robinson while working with a United States military think tank.

The scenarios add detail to a prospect currently haunting the world - one which Prime Minister Jacinda Ardern said during the election campaign needed United Nations focus.

"China is a critical part of resolving the dispute we have now - let's resolve it before it escalates even further."

Robinson devised the scenarios with retired US Navy rear admiral Stephen H Baker for the Centre for Defense Information, now known as the CDI Straus Military Reform Project.

Although done over 10 years ago, Robinson says the findings are as valid today.

The thrust of Robinson's argument is that North Korea is practising "crisis diplomacy" and using "weapons development in order to gain concessions, aid and favourable treaty outcomes with its prospective enemies".

"A first glance at North Korea's behavior might seem to show that the North is making threatening acts for no other reason than to disrupt the process of warming relations with South Korea, the United States, and its other neighbours.

North Korean leader Kim Jong Un responding to US President Donald Trump.

"However, re examination shows a careful policy of developing a threatening system or capability, and using that threat to gain attention - and hopefully concessions - from negotiating partners.

If it all went wrong, though, Robinson detailed a range of scenarios of how the war would unfold.

He said a conflict with North Korea would "bear little resemblance to what was seen in Afghanistan or Iraq".

But he said effective tools would be "exploited to the fullest", including special forces, precision-guided munitions, drones and the use of a strong intelligence, reconnaissance and surveillance network to guide rapid responses on the battlefield.

US military dominance would win out, he said.

"It is clear that in any conflict with North Korea, US forces will emerge victorious. The cost of the victory is another matter - indeed, there is a tremendous risk of loss of lives and massive destruction.

"Some military estimates put the civilian and military toll in the first day as high as one million.

Robinson spelled out the path to war in a range of scenarios.

North Korean fighters shoot down a US aircraft

There is precedent for this, with North Korea shooting down a US reconnaissance aircraft in 1969 off its coast. The aircraft crashed into the Sea of Japan and all 31 aboard died. There was no retaliation.

In 2003, Mig-23 and Mig-29 North Korean fighter aircraft got a missile lock on another US reconnaissance aircraft, against raising tensions.

During the recent heightened tensions, the US has flown bombers escorted by fighter jets close to North Korean airspace.

In the case of a fresh, successful attack on a US aircraft, Robinson said US ships and stealth bombers in the region could be expected to launch a devastating but focused attack on the airbase from which

A strike against nuclear facilities

Scenarios were studied as long ago as the mid-1990s and judged as likely to be successful, although not without consequences.

The key US aircraft would be stealth bomber and Northrop Grumman B-2 Spirit stealth bombers. Either could take off from Korean or Japanese staging bases and also the Andersen Air Force Base in Guam.

President Donald Trump speaking to military personnel at Fort Myer in Arlington, Virginia.

The stealth bomber would require a staging area in South Korea with a range of around 1000km. In 2003, as tensions heightened, Robinson says six stealth bombers were deployed to Kunsan Air Base in South Korea in a move seen by North Korea as "a major escalation and a rehearsal for invasion".

The B-2 gives greater flexibility for deployment with an 8000km range. It could be based out of the United States or Guam and easily reach targets in North Korea.

The downside of bombing nuclear production facilities is the likelihood of fallout. Prevailing winds could push radiation over Pyongyang, which has a population of 2.5 million people. It could spread further, across the Korean peninsula, or into neighbour China, Japan and Russia.

The other downside is that destroying nuclear production facilities would only stop more weapons being made. It does nothing for the weapons already manufactured and stored in a range of unknown locations.

People in Seoul watching footage of a missile launch by North Korea.

Ballistic missile strike on the United States

US intelligence reports have argued this is a possibility since 2003 although argued that it would be unlikely because North Korea would gain little and conceivably lose everything.

Robinson said casualties could range from a handful to hundreds of thousands killed and injured if a nuclear warhead struck a city.

If it were to happen, he said satellite reconnaissance would detect launch preparations early and could lead to an air strike on the facility.

Any missile attack "would probably bring near instant and overwhelming military retaliation".

North Korea attacks with artillery and rockets

It's just 40km from the border to Seoul and North Korea has developed specific capabilities to threaten the South Korean capital.

Robinson says an air attack on a significant North Korean target could bring about a counter-attack on Seoul using the "vast arsenal of artillery pieces, multiple rocket launchers, and missiles" stationed on the border.

Robinson says it could happen in conjunction with the detonation of a nuclear device smuggled into a South Korean city.

Even without a nuclear element, an estimated 5000 rounds could be unleashed in 24 hours costing hundreds of thousands of lives and causing billions of dollars in damage before the artillery and rocket batteries could be destroyed.

The political cost would be enormous and would signal an acceptance of "all out war" by North Korea, says Robinson.

Massive US air strikes

Efforts to destroy North Korea's artillery and rocket launchers to avoid damage to Seoul would be "very difficult", says Robinson.

An initial successful air strike would create pressure for a greater, more taxing attack aimed at removing North Korea's ability and will to retaliate.

"Critical to success would be achieving immediate, overwhelming destruction of the threat at the DMZ."

An American stealth bomber releases a B61-11 bunker-busting nuclear weapon. Photo / Supplied

Beyond the border threat, first night targets would include North Korea's air force, airfields, air defence and command and control facilities.

"The accuracy and lethality of the current weapon inventory would enable the United States to destroy a considerable number of the high-value targets throughout the country from the air.

"This strike would be devastatingly lethal and very intense. The goal would be to very quickly take away North Korea's will to fight and to stagger and isolate remaining ... formations, rendering them incapable of resisting."

Air dominance would be a key objective in the first 48-to-72 hours to allow control of the air space.

While the US could destroy much of the artillery and rockets threatening Seoul but the number and protections around them mean any attempt could be only partially successful.

And, says Robinson, North Korea might well anticipate the threat and attack first.

Full-scale war - North Korea attacks

Robinson says the layered defences at the border would pose an obstacle too great for North Korea to overcome.

After an opening, deadly artillery barrage, infantry supported by armour would head for Seoul through the two main routes south.

North Korean soldiers salute at Munsu Hill in Pyongyang, North Korea. Photo / AP

"A successful DPRK advance would require breaking multiple South Korean lines even as the South Korean reserves, capable of establishing further replacement defence lines, started to arrive."

The potential reinforcement numbers are huge. Not only does South Korea have numbers to call on but the US has substantial assets based in the region.

US aircraft would establish dominance quickly and cause massive damage on invading North Korean forces.

Once the advance was broken, the US and South Korea could be expected to counter-attack.

Robinson notes South Korea's "hesitancy" to consider conflict, likely linked to US estimates in 1994 that such a battle would cost the lives of 50,000 US service personnel and the lives of almost 500,000 South Korean military staff.

Full-scale war - the US and South Korea attack

Shock and awe - Robinson doesn't say the words but that's the thrust of a US pre-emptive strike.

Hundreds of targets would be hit in the first few hours and more than a thousand in the first night.

"It would be essential to stun key DPRK units, particularly those responsible for long-range artillery and aircraft, before they could react."

The 700,000-strong South Korean army would block routes to the south as air and artillery hammered the indentured North Korean military, much of it with inferior equipment.

The US has plotted on most North Korean underground shelters on the border and hundreds more to the north. Many key facilities have been "mapped out in precise detail".

"Weaponeers have decided what the optimum ordnance and approach is for each target set, from caving in entrances and exits to underground artillery batteries, to destroying MiG aircraft in revetted hangers, to causing reactors to collapse upon themselves to limit radioactive release."

Robinson says it was possible 90 per cent of the threat to South Korea could be eliminated in the first few days.

Such a "Blitzkrieg" could reduce allied casualties to an "acceptable minimum".

Try peace instead

Robinson says negotiation is the key because "war on the Korean Peninsula would likely mean hundreds of thousands killed and enormous damage".

"It must be avoided if at all possible."