What would happen if members of the European Union went to war?

Just wounding. Lets say Portugal and Spain had a severe disagreement over something and went to war. Portugal was the aggressor but Spain had a sizable military force (tanks and such) stationed at their borders and repetitively had fighter jets and bombers fly close and sometimes in Portugal airspace. What I want answered is this: 1. How would this effect the EU? 2. Who would win? 3. What would the reaction of the other members of the EU be? 4. Who would side with who?