EXPLAINING THE TREATY: Deaglán de Bréadún looks at the security implications of the treaty
For the first time, if the Nice Treaty is ratified, the principle of enhanced co-operation (also known as "flexibility") can be applied in the area of Common Foreign and Security Policy.
This means that, in the absence of unanimous agreement, a minimum number of eight member-states (currently there are 15) can take joint actions or policy positions.
However, to the disappointment of some Mediterranean member-states, military and defence matters are excluded.
If it concerns CFSP, an individual member-state can veto the establishment of a group for enhanced co-operation.
Nice also gives a legal, treaty basis to the EU's Political and Security Committee (PSC), already functioning and composed of ambassadors from each member-state but acting on instructions from their respective governments.
The PSC is responsible for day-to-day co-ordination of the EU's Common Foreign and Security Policy.
When the Rapid Reaction Force (RRF) comes on stream, the PSC will, under the guidance of the Council of Foreign Ministers, exercise "political control and strategic direction of crisis management operations" by the RRF.
Ireland has agreed to contribute 850 troops to the 60,000-strong RRF, which proceeds regardless of what happens to the Nice Treaty and is expected to be ready for action next year.
Its remit is based on the "Petersberg Tasks", named after Schloss Petersberg, near Bonn, where these tasks were first agreed in 1992. They are "humanitarian and rescue tasks, peacekeeping tasks and tasks of combat forces in crisis management, including peace-making".
The Government has said it will only approve participation in operations mandated by the UN.
The Amsterdam Treaty of 1997 envisaged a key role in EU defence for the Western European Union, founded in 1948 (Ireland has observer status). But, with the development of the RRF, the WEU has receded in importance and is effectively sidelined by Nice in favour of the PSC.