My starting point is that war must be forbidden within the EU : both war between nation states within the EU, and civil war within an EU country. There is a question of threshold, but we know war when we see it.

My hypothesis is that the operational aim is to separate the belligerents and/or put an end to atrocities against civilians, by the rapid application of overwhelming force.

Because it can present itself in all sorts of ambiguous ways, and because the question of sovereignty and legitimacy may be open to interpretation, the application of any such doctrine must be done in an ultimately arbitrary but rapid and decisive manner, leaving legal technicalities and political considerations for later.

This means that it can only be done by an existing nation state, not by any EU mechanism.

As far as I can see, only two nation states have the military capacity to intervene anywhere in Europe with overwhelming force. If it were a matter of two EU states fighting each other, I can imagine that the two would intervene in  a concerted manner. If it were a civil war, I can't imagine the UK putting boots on the ground.

_{It is rightly acknowledged that people of faith have no monopoly of virtue - Queen Elizabeth II}