It’s always assumed that defence attaches play some kind of role in gathering intelligence which is generally tolerated by their host countries.

They could be gathering data about weapons production for instance or shipbuilding – but there is a line they are expected not to cross.

On Wednesday, the Home Office expelled a Russian defence attache. So the assumption now is that the Russians have broken the rules of the game.

But the British government will have thought long and hard before their decision. It doesn’t come without cost.

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In their day jobs, defence attaches play a crucial role in liaising with their hosts.

That can be very important in terms of avoiding misunderstandings that could lead to unnecessary and dangerous escalation.

Losing Russia’s defence attache in London deprives the UK of one more line of liaison at a time when clear understanding and clarification could be essential to avoiding deepening conflict.

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That is not ideal at a time of war.

What is not clear is how much these moves are connected specifically to the conduct of the Russians involved – or whether they should actually be seen as another chapter in the diplomatic war between Russia and the UK.

The UK has undoubtedly weakened Russia’s ability to spy in the UK and gather intelligence but these measures will also affect Britain’s ability to predict and avoid potential escalation.

That will be made even worse if Russia – as may be expected – responds with like-for-like retaliation.