As Attorney general, I spent quite some time wrestling with the reach, and limits, of international law.

Leaving aside international criminal law for a moment, unlike domestic law, international law is largely based on convention, or on treaties which set out rules nation states have agreed to be bound by.

Unlike domestic law, the international law courts which exist only have authority over the states who have signed up to their jurisdiction.

International law can often feel weak, ephemeral or unenforceable, but it does matter.

It remains a framework within which the behaviour of states can be judged and where breaches provide a basis for international action against a transgressor.

Very few states are prepared to say they are not bound by it, exemplified by regular attempts by countries of all sorts to claim, however unconvincingly, that they are in compliance with international law when they act.

In my view, international law, imperfect as it undoubtedly is, remains our best chance of constraining the bad behaviour of nation states and of holding them to account for it when it happens.

This is why the recent acts and statements of the President of the United States are of such concern.

The use of force by one nation against another is not prohibited altogether by international law (usually encapsulated in the United Nations Charter), but it is only deemed acceptable in certain circumstances.

That means a nation using or threatening force must identify which of those circumstances it seeks to rely on to justify its behaviour.

Explanation of legitimacy is not optional, and whether in relation to Venezuela or especially to Greenland, we have seen little of it from the US Administration.

On Greenland, it is hard to imagine what international law justification there could be for the threat or use of force against a NATO ally and I think the Government has been right to say so.

What President Trump has said clearly however is that he remains unconvinced that European countries, including the UK, are doing enough to defend their own interests and those of NATO.

We should listen. Whether the US is persuadable, with increased European contribution, to remain committed to NATO (as I hope and believe) or whether he intends to walk away from it, the logic is the same.

We have to spend more on our own defence and we have to do it soon. Probably we should have done it sooner but that is no reason not to do it now.

The fairly vague commitments the Government has made on this so far need to be translated onto real money and plans because the world is getting more dangerous, more quickly.

And it isn't just President Trump who is paying close attention, it is also President Putin and others who wish to see Western democracies weak and divided and who wish to see NATO fail.