Funny enough, however hard you might have spent the two minutes you didn't help a tiny bit. We were speaking of ASSOCIATION AGREEMENT, not of MEMBERSHIP. Here is the text of the former:
http://ukraine-eu.mfa.gov.ua/en/page/open/id/2900
No mentioning of any military aspect.
But even if it were about EU membership and a possible attack on a member state - the EU doesn't have any (joined) armed forces. Evidently, since most of the EU states are also NATO states it would be NATO article 5 that would be enforced in such a case.
You can read the agreement and figure out yourself if it is that costly or resposibilities-fraught as experts believe.
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