In the coming days, there are three possible triggers for war with North Korea that need to be carefully watched.

The first possible trigger is a declaration of war by North Korea, especially since the United States has made clear it has not declared war. The idea that countries would formally declare war against each other, before commencing hostilities, is a relic of the early 20th century. Although remnants of the practice remain, it was largely outdated by the second world war as the military advantages of surprise as the Japanese attack on Pearl Harbour and the Nazi attack on Soviet Russia, made clear.

After the second world war, the United Nations hoped that all members would refrain from the threat or use of force against the territorial integrity or political independence of any state, to which Declarations of War by individual states would become redundant. However, when the North Korean armed forces advanced over the 38th parallel on June 25, 1950, without a declaration of war, it was shown how in vain this hope was.

The response to this act of aggression by North Korea was the 84th resolution (pdf) of the Security Council (when the Soviet Union was absent from the vote) to defend South Korea under the UN flag but with the leadership of the US.

Today, the situation is even more complicated as the North Korean rhetoric of declaring war is not uncommon. Following the 2013 sanctions (pdf) approved by the Security Council against North Korea for their nuclear test, Kim Jong-un promised a pre-emptive strike against the US with nuclear weapons. This was followed by a "Full War Declaration Statement". This was all part of their assertions that North Korea had scrapped the armistice that ended the first Korean War in 1953.

To show their determination in 2013, North Korea also cut the hotline that enabled direct communication between North and South Korea. Although the hotline was reconnected a few months later, when South Korea closed down the joint Kaesong industrial complex following Kim Jung-un's fourth nuclear test in early 2016, North Korea condemned the act as a Declaration of War, and then cut the hotline again.

READ MORE: North Korea tensions - All the latest updates

Cutting the hotline is more dangerous than the rhetoric. Hotlines prevent accidental war. South Korea, which has a hotline to China, has been trying to have its hotline to North Korea reconnected. However, the line that is really needed is one between North Korea and Washington. Such best practice has been evident since 1963, following the Cuban Missile Crisis, when the two superpowers recognised the necessity to be able to talk directly, at short notice, whenever required so as not to stumble into nuclear war.

Possible detonation of a nuclear device

The second possible trigger to watch for is the North Korea threat of a possible detonation of a nuclear device in the atmosphere over the Pacific. Although the aesthetics of such an act would shock the world as humanity has not seen a nuclear airburst since that done by China in 1980, this would not be the beginning of hostilities. Such an act would violate international environmental law, not the laws of war and peace. Kim will start a war if he detonates his device over or within someone else's territory, including the ocean spaces that they control, such as with the American land and ocean territory of Guam.

Kim will start a war if he detonates his device over or within someone else's territory, including the ocean spaces that they control, such as with the American land and ocean territory of Guam.

However, if he exploded it in international territory, such as the high seas, he faces different rules, such as when Australia and New Zealand took France to the International Court of Justice (pdf) after French atmospheric testing caused radiation pollution to fall on them, downwind. It was for this reason of pollution that most of the global community concluded an international agreement prohibiting such atmospheric nuclear testing. Although North Korea is not a signatory to this agreement, the same international rules (pdf) about not causing significant environmental damage to other nations still apply.

Shooting down an aircraft

The third possible trigger to watch for is the North Korea threat to shoot down aircraft in international airspace (as in, mirroring the territorial sea, 12 nautical miles/22.2km out from the land). Previously in 1969, North Korea did shoot down an American spy plane (pdf), killing all 31 members aboard when it was operating in international airspace. At that point, President Nixon did not respond with violence due to a fear of how the Soviet Union and China would react.

Today, as the over 60 times that Russian military aircraft have flown close to Alaska or down past the edge of Western Europe in the past 10 years have shown, no matter how unpleasant such acts may be, such planes may be intercepted and followed, but they may not be shot down if they do not cross into territorial airspace. To ensure that no mistakes are made in this carefully choreographed sabre rattling, certain rules need to apply - primarily, the planes should not be invisible. Where possible, transponders and radios should be on, and (as the US appears to be doing) flight plans disclosed in advance.

If Kim decides to take down one of the American planes flying in international airspace, as his grandfather Kim Il-sung did in earlier times, he would be gambling against the odds that President Trump will not respond with violence.

Alexander Gillespie is professor of international law at the University of Waikato, New Zealand. His research focuses on laws of war and armed conflict. He is the author of the three-volume set "A History of the Laws of War" and the three-volume set "The Causes of War".

The views expressed in this article are the author's own and do not necessarily reflect Al Jazeera's editorial policy.