The Nice Treaty does not alter in any substantial way the provisions of the Amsterdam treaty in relation to European security and defence. Enhanced co-operation or flexibility has been agreed as a procedure which could apply in the common foreign and security policy, subject to unanimity in the decision establishing the group and involvement by a minimum of eight countries. However, the relevant article of the treaty, Article J, states quite clearly that closer co-operation does not apply to matters having security and defence implications.
Participation in Petersberg missions will continue to be for governments to decide on a voluntary and sovereign basis according to their respective national arrangements. In Ireland's case, that means a UN mandate and approval by Dáil Éireann.