Originally Posted by
petit plateau
I think you've now sussed out the point. If a party elects to become a co-belligerent then NATO Art 5 no longer applies.
I would like to hope UN applies, but that hope died stillborn back in 1945 when circumstances meant that the same problems as bedevilled the League Of Nations were baked in.
My take. If Poland sent troops to Ukraine then they are fair game. Russia attacks Poland then Article 5 applies.