In an extraordinary escalation of his bid to claim Greenland, US President Donald Trump has threatened eight European countries – Denmark, Sweden, Norway, Finland, Great Britain, France and Germany and the Netherlands – with a 10% tariff on all goods coming into the United States from February 1 until he is able to buy the semi-autonomous Danish territory. That tariff will then increase to 25% on June 1.

On the one hand, Greenland is potentially rich in raw materials and rare earth minerals, highly desirable for US tech giants who control key levers of power in Washington. On the other, Trump claims it is necessary for national security.

Greenland is part of a sovereign country, Denmark, and any offensive action against it would constitute an act of aggression.

In the past few days, a small number of European troops have arrived in Greenland to bolster its defences. Trump’s recalcitrant stance has sent shockwaves across Europe, which is now questioning the future of NATO.

So what might happen now?

Aggressive wars are illegal under international law. Under the UN Charter, the use of force is lawful only when

1. authorised by the UN Security Council under Chapter VII

2. as self-defence under Article 51 in response to an armed attack.

In this case, as the US’ claims have no backing from the UNSC, its use of force would require a pretext or provocation that would allow self-defence – in other words, an attack or imminent attack.

As I always tell my students in international law, these must be scrutinised carefully as there is long, chequered history behind such claims. The key problem are “false flags” by........

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