One thing certain about wars is that they never go as planned. WW2 being the biggest example.
Lets entertain ourselves with following scenario:
Russian fighter-bombers wade into Polish airspace running away from Ukrainian airforce, and launches a missile in anger at Polish MiG.
Poland president then instantly calls art. 5. What happens next?
An art.5 invokation in no laughing matter. If the US will render it hollow, NATO falls apart in an instant. It would not be like art.5 invokation by Bush: the circumstances are completely different.
Even before Trump, NATO's fundamentals been squeaky. Allgedly, US has secret arrangements with core NATO countries to renege on honouring art.5 if invoked by Turkey.
If the above is indeed true, then there us no wonder why I felt that EU started entertaining the idea of "EuroCorps" right around that time.