What would the EU do if an EU member declared war on another EU member? I am wondering what would happen if a country like France attacked a country like Cyprus, what would the EU do in that situation? Is there a sort of mechanism in place that would guide the EU in its decision?
According to the provisions in the Treaty on European Union, the EU itself wouldn't be able to do much directly, however it could delegate a conflict resolution task to some number of member states. Further, all EU members would be obligated to provide aid to the attacked member state.
Title V, Chapter 2, Section 2 of the Treaty on European Union states the following:
- The common security and defence policy shall be an integral part of the common foreign and security policy. It shall provide the Union with an operational capacity drawing on civilian and military assets. The Union may use them on missions outside the Union for peace-keeping, conflict prevention and strengthening international security in accordance with the principles of the United Nations Charter. The performance of these tasks shall be undertaken using capabilities provided by the Member States.
- The Council may entrust the execution of a task, within the Union framework, to a group of Member States in order to protect the Union's values and serve its interests. The execution of such a task shall be governed by Article 44.
- 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 shall not prejudice the specific character of the security and defence policy of certain Member States.
I would expect the EU to work very hard to try to mediate and resolve any dispute between members long before it got to the point where one member was to declare war on another.
If a large and influential nation like France was to attack a smaller nation like Cyprus 'out of the blue', it would probably be an existential threat to the EU itself, as it would be a huge blow to the spirit of cooperation and trust that the EU is founded on. Other members would be forced to either take sides; reprimand or punish France somehow (which would be difficult and cause a lot of internal tension); or otherwise turn a blind eye as France 'has a go' at Cyprus. None of those options would be very appealing or positive for EU cohesion.
According to the treaties, they would have to defend both members. So they would fight for one member on Monday, Wednesday, and Friday, and for the other member on Tuesday, Thursday, and Saturday. Sunday could be used to plan the schedule for the next week.
Seriously, two EU members fighting a war is quite incomprehensible for logistical reasons, as well as political ones. The EU members have already integrated their armed forces to a degree that only small expeditionary detachments could fight independently. Anything larger than that would run into the problems that some specialized capability is only provided by a few member nations.
NATO has nine rapid deployable corps headquarters in Europe: ARRC, EUROCORPS, Northeast, Italy, Turkey, German-Netherlands, Spain, France, Greece. Turkey isn't EU, and the EU has some members who are not NATO, but the big ones are.
Not sure about the Spain and France (and Turkey) headquarters, but the rest are multinational.