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Since Maastricht the European Union has been developing a military element. Firstly with the Common Foreign and Security Policy, then (via the Western European Union vehicle) the European Security and Defence Identity and European Security and Defence Policy. The WEU integrated with the EU in 1999.

The first missions were deployed in 2003 and the Lisbon Treaty introduced the Common Security and Defence Policy which in turn introduced the European External Action Service and Permanent Structured Cooperation in Defence.

What are the arguments for militarising the European Union - ostensibly started as a trade bloc - in this way?

The EEAS has an informative piece on the Common Security and Defence Policy. The arguments you ask about can be found by looking at what the policy aims to do: the Petersberg Tasks (quoting from the previous link by EEAS).

These tasks, to me, seem more outgoing, whereas NATO is primarily aimed at defending member states. The reason for doing this through the EU rather than as individual member states is the same as why anything is done through the EU (as opposed to individual member states): because it's more efficient to work together.

The Petersberg tasks formed an integral part of the then European Security and Defence Policy (ESDP) - now Common Security and Defence Policy (CSDP) - and defined the spectrum of military actions/functions that the European Union can undertake in its crisis management operations.

The Petersberg tasks were first agreed upon at the June 1992 Western European Union (WEU) Council of Ministers near Bonn, Germany. Article II.4 of the subsequent ministerial declaration outlined the following three purposes for which military units could be deployed:

• humanitarian and rescue tasks;




• peacekeeping tasks;




• tasks of combat forces in crisis management, including peacemaking.

The term ‘peacemaking’ was adopted as a consensual solution and as a synonym for ‘peace-enforcement’. The Petersberg tasks were subsequently incorporated into Article 17 of the Treaty of the European Union (TEU) through the Treaty of Amsterdam.

The 2009 Treaty of Lisbon (TEU Art. 42) then further expanded these tasks to include:

• humanitarian and rescue tasks;


• conflict prevention and peace-keeping tasks;


• tasks of combat forces in crisis management, including peacemaking;


• joint disarmament operations;


• military advice and assistance tasks;


• post-conflict stabilisation tasks.

Attribution: https://eeas.europa.eu/topics/common-security-and-defence-policy-csdp/5388/shaping-common-security-and-defence-policy_en#The+Petersberg+Tasks

Currently the EU members promise to defend each other against armed aggression in Article 42(7) TEU.

If I understand you correctly, you see this as an change of the avowed original purpose of the EU (note the word "ostensibly" used by you). I see it as a perfectly logical development of a union with shared democratic values -- a part of the EU being occupied by a third party would certainly hinder free trade in goods and services!

• While it would be possible to maintain this promise on a political level without military preparations for a common defense, it seems logical to prepare for joint action.


• Most but not all EU members are also NATO members, and vice versa. Traditionally the US maintained the capability to act both independently and as a part of NATO while the other NATO partners would have trouble to handle a major theater war on their own. The US complains about this imbalance. (I'm not sure that they would be happy if the EU had and used an independent capability, but that's not the question here.)
  


• Military integration under the NATO umbrella has reached a point where most EU members would be unable to act without the others, except for small expeditionary forces.


• It would a logical extension of the free trade rules to have EU-wide open competitive bidding on military purchases. A small step from there to have a joint procurement process.