I've done a bit of rummaging - there are two Lisbon treaty clauses that I think you could be referring to. I find this fascinating so feel free to skip everyone who has something else to do on a Friday eve
Art 42.7 “If a Member State is the victim of armed aggression on its territory, the other Member States shall have towards it an obligation of aid and assistance by all the means in their power, in accordance with article 51 of the United Nations charter”
This refers to member states (rather than the EU) acting on their own behalf but together, and through a vote in the EP. It is worth noting it doesn't override national security/defence concerns, through "“shall not prejudice the specific character of the security and defence policy of certain Member States”".
This article was requested by Greece as Turkey is part of NATO and their were fearful that NATO may not protect them. It has been used, by France in response to last year's terror attacks and to garner support for its actions against IS. It is also worth noting that there was considerable interpretation by each member state as to what their "assistance" might mean.
And then there is this:
Art 222 The Union and its Member States shall act jointly in a spirit of solidarity if a Member State is the object of a terrorist attack or the victim of a natural or man-made disaster. The Union shall mobilise all the instruments at its disposal, including the military resources made available by the Member States, to:
(a) - prevent the terrorist threat in the territory of the Member States;
- protect democratic institutions and the civilian population from any terrorist attack;
- assist a Member State in its territory, at the request of its political authorities, in the event of a terrorist attack;
(b) assist a Member State in its territory, at the request of its political authorities, in the event of a natural or man-made disaster.
Note that this refers to terrorism and "manmade disaster" rather than acts of armed aggression as 42.7 does. Note also that council members can abstain, (per Article 31(1) of Treaty of European Union - at least I think that's what that says) which is a "constructive abstension" and means you don't have to take part in the action.
So it seems less certain that you have any requirement to take part in any action that you wouldn't want to. I will have a look at that link though - I guess another good point is that we will need to define how we interact with EU and European defence sounds like more bloody bureaucracy in future.
RIGHT, wine time.