For example, say that Finland would be attacked. Finnish citizens has the right to move - to say Paris or London - within the EU framework. The finnish government would demand help in the council of ministers and finnish MEP would demand condemnation from the European parliament. While there is no formal military alliance having a member country conquered by a non-member would wreak havoc on the EU structure.
And EU has some of the strongest militaries in the world. In spending Britain and France regularly ends up on top five.
So the norwegian troops would have to be forced out of Finland. A vote for PES is a vote for EPP! A vote for EPP is a vote for PES! Support the coalition, vote EPP-PES in 2009!