When it comes to security and defence, the decision has to be unanimous. If one member state, even a tiny one like Luxembourg, says no, then it doesn't happen. Afraid not, Juncker wants to extend the EU competences
to Defence, and thus it will be under QMV not the veto.
There are plans already drawn up for the establishment of an EU military HQ, and indeed, an operational HQ is already in existence and in use.
Whilst I can't see a dedicated EU military existing because it will cost too much, and one of the two effective militaries within the EU will shortly be leaving the project, there is the political will there for it, Merkel has announced that Germany is to increase defence spending (won't be hard, given current levels) because the US can't be relied on, ergo, she doesn't think NATO can be relied on, and the EU should go it alone.