• Alexander Gillespie is a professor of law at Waikato University.

North Korea is threatening to detonate a nuclear device in the atmosphere over the Pacific. Although this would be visually terrifying, it would not be a declaration of war. Rather, it would be a further acceleration of speed between two men on a collision course, both accusing the other of being insane.

The last test of a nuclear device in the atmosphere was by China in 1980. Most other nuclear-armed countries had stopped testing in this way decades earlier due to the radiation pollution the tests caused on countries downwind. It was for this reason New Zealand took France to the International Court of Justice in the 1970s, and most of the global community concluded an international agreement prohibiting atmospheric nuclear testing. North Korea is not a signatory to this agreement.

If Kim Jong Un does his atmospheric display he will violate international environmental law, not the laws of war and peace. He will violate the laws of peace if he detonates his device over or within someone else's territory, including the ocean spaces that they control. If Kim puts one of his missiles into the ocean area of the American territory of Guam, Donald Trump would be within reason to construe this as an act of war and retaliate.

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If Kim delivers his threatened atmospheric blast to the Pacific by missile, he will be proving to the world that his intercontinental projectiles can deliver nuclear payloads anywhere on the planet. In so doing, Kim will continue to violate the norms which keep peace by sending his nuclear device over another country, such as Japan, without their consent.

The hypocrisy and bullying of Japan by North Korea on this point is extreme. If Donald Trump fired a missile over North Korean territory without their consent, Kim would likely respond with violence.

If Kim prefers a different delivery method, such as by submarine, for his proposed nuclear air-burst, he will still be showing the world that nowhere is safe. His submarines could surface off the coast of New York, as much as they could place themselves behind any missile defence system that is currently pointed towards North Korea.

Kim is threatening to escalate the tensions because the sanctions are getting tighter, the rhetoric more hateful, and Donald Trump and the international community are not backing down.

In essence, backing down entails allowing North Korea to keep its nuclear weaponry. North Korea wants to be treated exactly the same as India, Pakistan and probably Israel, each of which has also successfully joined the list of super-powers with nuclear weapons and been allowed to keep them, despite much grumbling by the global community.

The problem with North Korea being able to keep its nuclear weapons is twofold. First, North Korea is a not a signatory to the convention on the non-proliferation of nuclear weapons, and may pass on nuclear technology to other countries or organisations that are willing to pay for it. Second, it will fundamentally upset the balance of power in the region as South Korea and Japan will want their own nuclear deterrent as counterweights to those of North Korea.

The difficulty here is that more nuclear weapons in more hands makes the world more dangerous not less. In particular, nothing annoys China more than the thought of a country like Japan bristling with its own nuclear weapons and advanced missile defence systems.

Everything turns on China. It has made clear that if North Korea is attacked first by the United States, it will stand and fight with its North Korean ally. However, if North Korea starts the war, China will not intervene. This difference is critical. A regional war without China involved could have a death toll of a million. A global war with China involved could have a death toll of billions.

Donald Trump knows this and it is why he is trying very hard to keep China, along with the other members of the Security Council acting in unison, slowly tightening the sanctions, through the United Nations, around the neck of North Korea.

Despite sanctions being progressively ratcheted up on North Korea since 2006, we are not yet at maximum pressure. Although it is questionable how much influence further sanctions will have upon a communist tyranny with one of worst human rights records on the planet, the tightening must continue.

The next steps are complete diplomatic isolation of the country; stopping all North Korean citizens leaving the country for sports, cultural exchange or the provision of labour; and finally, reducing the cap on oil imports to zero. This last measure will be most effective, as Kim will find his tanks and planes do not run well on coal.