Of course they will help Boris from Bulgaria because allowing Bulgaria to be taken over the Russians will be one step closer to the back door of their own country. Eastern Europe is a buffer zone that works as long as they are not conquered.
This is why this shit is not going to fly in Poland, our government will do everything in their power to get as much US troops on the ground as possible, at the cost of being USA gofer in Iraq/Afganistan/Iran/wherever POTUS fancies this week.
Germany has already been shown going back to their old tricks, scheming another Molotov-Ribbentrop behind whole EUs back with Nord Stream, and especially Nord Stream 2. Paradoxically US is the only country with some backbone to oppose this german-Putin deal, with most western EU countries sitting silent.
One could reasonably argue that Ukraine simply wasn't part of the EU, and if Russia threatened Poland it would be a different matter. That it is the EU membership, not the buffer zone, that is important.
Another possible interpretation is EU would have wanted to intervene, but its current political structure proved too deliberative to react before it was too late. That would imply an EU army would also need political reform.