A further question we often hear is whether an EU army is possible. The answer is found in Article 42 of the Treaty on European Union:
PROVISIONS ON THE COMMON SECURITY AND DEFENCE POLICY
(ex Article 17 TEU)
1. 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.
2. The common security and defence policy shall include the progressive framing of a common Union defence policy. This will lead to a common defence, when the European Council, acting unanimously, so decides. It shall in that case recommend to the Member States the adoption of such a decision in accordance with their respective constitutional requirements.
The policy of the Union in accordance with this Section shall not prejudice the specific character of the security and defence policy of certain Member States and shall respect the obligations of certain Member States, which see their common defence realised in the North Atlantic Treaty Organisation (NATO), under the North Atlantic Treaty and be compatible with the common security and defence policy established within that framework.
We see from this section of the treaty that, again, unanimous agreement is required in order to pursue a "common defence policy". The European Council consists of all the elected leaders of EU member state governments (so, the UK Prime Minister for example) plus the President of the Council (currently Donald Tusk) and the President of the European Commission. It's quite clear that an "EU army" could not happen unless we agree to it, providing we remain a member of the EU.