Assuming the UK and EU stick to their respective red lines on sovereignty and autonomy, a future security partnership is likely to be modeled technically on ones the EU already has. In this case, even an EU-UK security partnership unprecedented in ambition will fall well short of the cooperation reserved for EU members—at a time when that cooperation is becoming more consequential in the global arena.

Security does and will continue to play a role in the Brexit negotiations. The main risk is that an acrimonious divorce could sour discussions to develop a strong and comprehensive EU-UK security partnership. It is in both the British and the European interest that this does not occur.

Although EU strategic autonomy and European strategic autonomy are different issues, one is likely the condition for the other, and vice versa. From outside the European Union, the United Kingdom would have a strong case to make that European strategic autonomy is a prerequisite of European security, and indeed a necessary precondition of EU strategic autonomy.

Britain is leaving the EU just as the EU is getting serious about defence cooperation; indeed, the fact that it’s getting serious about defence cooperation is linked to the departure of Britain, which consistently opposed such steps. With the UK out of the picture, the remaining EU members can integrate their defence at liberty. That leaves the UK the sole outsider, a loser as European allies form the closer union that’s necessary in order to maximise the region’s collective might.

Since the 1980s, British-German cooperation was referred to by observers in both London and Bonn (later Berlin), as the ‘quiet alliance’ (stille Allianz). A partnership that was running smoothly without seeking or receiving undue attention. Within the triangle of...