There were two highly significant meetings for Britain in Brussels on Monday. Britain’s role in the first meeting was sensible and constructive. But its role in the second was the reverse – foolish and destructive. Taken together, the meetings illustrate the international wound that modern Britain is inflicting on itself – and on Europe – as a result of the Brexit vote.

In the first meeting, EU foreign ministers including the British foreign secretary tried to put the final touches to a statement on Russia in the wake of the Skripal poisoning and the re-election of Vladimir Putin. There were disagreements and differences of emphasis, but the outcome was successful, building another brick into the wall. The meeting was the latest in a series of international efforts since the crimes in Salisbury to rally allies behind an agreed response to the Skripal case. Those efforts have included the joint statement by the UK, France, Germany and the US last week, the lobbying in Washington for clearer Trump administration backing, and a Nato resolution of support for Britain.

In the second meeting, the EU and British negotiators announced a deal on the terms of a 21-month Brexit transition period, during which the UK will continue to benefit from the EU’s single market and customs union provisions, while losing its role in decision-making bodies. The good news is that the transition arrangements keep the UK aligned with the EU on common economic and trade issues until the end of 2020 – that’s better than the reverse. The bad news is that the government’s attempt to portray this as the “implementation” of a coherent new relationship between Britain and the EU now lies in pieces. The transition deal once again poses the basic question: what is the point of this dealignment for Britain?

Faced with crises like the European relationship with Russia and the very different British relationship with the EU, a sensible modern Britain would attempt to adopt a consistent strategic approach. There would be an underlying common factor in both approaches: strategic recognition of the importance of upholding international rules in Europe for the benefit of all nation states. The long-term purpose is to strengthen and reward a rules-based international order and to penalise aggressive disruption like Mr Putin’s.

Britain’s reaction to the Skripal poisoning has been broadly correct. It is vital to assert collectively that such attempted killings, especially when they involve chemical agents, are beyond the pale. All policy actions should aim at showing the cost of stepping outside such rules. It is important to use agreed enforcement processes, to get the tone right and to avoid hypocrisy. But the threat is extremely serious. By and large, Britain has followed that path.

This matters more widely in the wake of Mr Putin’s re-election because, although his win in Sunday’s contest was wholly expected and predictable, it is a reminder that liberal democracies must play a long game in the contest with authoritarian regimes. Dealing with Russia will remain difficult for years to come. Strategic alliances between the European nations are vital if the long process of defending liberal democracy against Russian and other threats is to be successful. In this contest, European unity is essential to prevent individual European countries becoming playgrounds for Mr Putin.

There can be no doubt that Britain would make a more effective contribution to that common approach if Europe saw it as a reliable ally. Yet that is not the case because of Brexit. Brexit does not just offend allies and neighbours, making them less likely to respond when Britain asks for help, as it has been doing. It also actively weakens the rules-based liberal order by making Britain’s adherence to it less certain and by weakening the system’s overall authority. This is precisely what Theresa May’s government is trying to do in the Brexit negotiations, and precisely what Michel Barnier and his colleagues are seeking to defend. Britain is right to ask for support over the Skripal affair. But the principle of all for one and one for all is a reciprocal principle. That’s why Brexit is such a threat not just to Britain itself, but also to Europe’s essential and shared security interests.