The EU DOES have a military peacekeeping force (which the UK was part of until Brexit).
I have a goddam photo i took myself of a number of soliders from it on my wall. They exist already.
I'm guessing we'd be looking at an upscaling of that, probably with the UK back on board. It wound me up during the referendum campaign that people were oblivious to it already existing and instead chose to go on about who led it etc etc. If NATO can manage international military cooperation, i never understood why a similar EU one ever scared people as much as it did.
It is not remotely new or unprecedented. I've mentioned it several times on these threads before.
en.m.wikipedia.org/wiki/Operation_Althea
Current name: Operation Althea. Formerly EUFOR.
Originally NATO lead after Dayton Agreement then EU lead but not exclusively EU countries.
Currently Albania, Austria, Bulgaria, Chile, Czech Republic, France, Greece, Hungary, Ireland, Italy, Poland, Portugal, North Macedonia, Romania, Slovakia, Slovenia, Spain, Switzerland and Turkey.
Now whether a similar arrangement would work or whether Russia would think it laughable is another matter. I think the UK's involvement would be necessary tbh to be taken seriously.
It is the closest template I can think of and does sound not dissimilar to noises being made by Ukraine and West.