Ratified (2000) by all [then] 15 members of the European Union. The last country to ratify it was Ireland which had refused it the first time. This country accepted the Treaty during their second referendum on the matter the 19 october 2002 by 62.89%. The Treaty of Nice replaced the Treaty of Amsterdam: See details on Amsterdam Treaty.
The Intergovernmental Conference faced the main challenge of setting the bases of an Union enlarged towards the East. Besides the twelve countries (Cyprus, Czech Republic, Estonia, Hungary, Poland, Eslovenia, in the first wave, and Bulgaria, Letonia, Latvia, Malta, Rumania and Slovakia, in the second phase) that had already started negotiations, and Turkey was a main candidate for future consideration. The application of this euroasiatic and Muslim country was officially admitted in the Helsinki European Council, held in december 1999, although negotiations were put off until Ankara's government complied with the accession criteria, regarding respect of human rights and protection of minorities. Evidently, the Kurd problem was in EU negotiators' mind.
This Treaty allowed the European Enlargement. On 9th October 2002, the Commission recommended to close negociations with Cyprus, the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Malta, Poland, the Slovak Republic and Slovenia. The objective was that the first group of new members would join the EU in time for the elections to the European Parliament scheduled for June 2004.The following countries became officially members of the EU on 1st May 2004: Cyprus, Czech Rep., Estonia, Hungary, Latvia, Lithuania, Malta, Poland, Slovakia & Slovenia.
Three years later, Romania and Bulgaria joined the European Union on January 1st 2007. The European Commission could have advised that they get the formal go-ahead on their May 2006 meeting. Instead it hesitated until a further review in the autumn of 2006 showed proper progress on reforms.
Turkey, Croatia and Macedonia had been accepted as candidates, but their acceptance as full members into the EU were forecasted to take place sometime after 2008.
However, The Treaty of Lisbon (initially known as the Reform Treaty) was approved as an international agreement that amends the two treaties which comprise the constitutional basis of European Union (EU). The Lisbon Treaty was signed by the EU member states on 13 December 2007, and entered into force on 1 December 2009. The stated aim of this treaty was "to complete the process started by the Treaty of Amsterdam  and by the Treaty of Nice with a view to enhancing the efficiency and democratic legitimacy of the Union and to improving the coherence of its action."
On the path to draft and approve the Treaty of Nice, observers had, almost unanimously, affirmed that national interest outweighed Europe's during the arduous debate that took place at the Nice European Council. However, after a lot of give and take, Union's tradition held on: an agreement was reached.
The main points of the Treaty of Nice that ammends the previous treaties:
- The bitter debate among big and small countries for the re-weighting of vote in the Council, represented in the Iberian context by the struggle between Portugal and Spain, rose the greater tensions. Finally, a new re-weighting of vote for the current and the future member States was reached. This new distribution of power came into force on 1 January 2005 in the case of current members. The new system gives 29 votes to the Four Big Countries (Germany, France, United Kingdom and Italy). In spite of the demographic imbalance, parity between France and Germany is maintained. Spain got 27 votes, the same as Poland when joining the Union. The rest of the countries got progressively less votes up to Malta, whose weight will be three votes in the Council.
- A complicate system of majorities and minorities was set up. It provides three different ways to block any decision of the Council:
1. When the Union have 27 members, the whole number of votes of the Council will be 345. The threshold for the qualified majority is set at 255 and a minority of 88 votes would veto any resolution. That means that three big and one small countries will be always able to impede any decision.
2. A simple majority of member States opposition will always prevent a decision from being passed by a qualified majority.
3. Finally, a demographic verification clause was adopted to give more power to more populated Germany. To get a qualified majority is necessary that a proposal be endorsed at least by 62% of the Union's population. It means that Germany needs the support of two big nations to veto whatever decision. The rest of the big countries need the participation of all the four big to exercise the veto.
- The European Parliament will consist of 732 seats, instead of current 626. Germany will elect 99 MPs, the rest of the big countries 72, Spain and Poland 50. Seats in the Parliament have been used to make up for disparities in the re-weighting of votes in the Council.
- In 2005, countries that currently have two commissioners (Germany, France, United Kingdom, Italy and Europe) will have one. When the Union be composed by 27 members, a definitive number of commissioners will be unanimously decided. The number will be inferior to 27 and will be set up a a fair system of rotation that balance the composition of the Commission, taking into account the different demographic weight of the countries and the diverse geographic areas of our continent. On this subject, one of the main battlefields between big and small countries, a definitive solution was not reached, although the features of a future agreement were outlined.
- Competencies of the President of the European Commission were enhanced. Henceforth, he will be elected by qualified majority and his or her appointment will be ratified by the European Parliament.
- The subjects on a decision by qualified majority can be adopted increase (about forty, most of them technical ones). However, governments' veto is maintained in subjects that affected them in a high degree, such as cohesion (Spain) , tax system (Britain), asylum and immigration (Germany) or free trade in cultural an audiovisual sphere (France).
- The possibility of a reinforced cooperation among member States in fields related to further integration was set up. Reinforced cooperation must meet some conditions:
o At least, eight countries must participate to start this sort of closer cooperation.
o The following aspects are excluded of this possibility: Community policies, affairs related to Schengen, and anything else that affected the common market, defence and weapon industry affairs.On 26 February 2001, the European leaders, gathered again in the Côte d'Azur capital, proceed to sign the Treaty of Nice. The Treaty dealt mainly with the institutional adaptations required for the expansion of the Union to 25 Member States. These issues remained unresolved with the Treaty of Amsterdam and they provided the background for the one of the most difficult negotiations in the history of the Union. The Treaty came into force on 2 February 2003.