EU would have to deploy immediately at least two heavily armed divisions to be able to protect Polish-Belarusian border (which is in reality Polish-Russian border and would be first attack direction to cut supply chains from Polans) and attack Kaliningrad, what is the key in Baltic countries defense.
Nothing happens in EU without Germany agreement, so the question is: will German army going to engage? Germany is busy cutting trade deal with China and starting Nord Stream 2 which would provide Russian gas for Germany, so the answer is clear.