The entry into force of the treaty was in doubt for a time, after its initial rejection by Irish voters in a referendum in June 2001.

As a transitional measure it specified that after 1 January 2005, Germany, France, the United Kingdom, Italy and Spain would each give up their second Commissioner.

In response to the failed sanctions against Austria following a coalition including Jörg Haider's party having come to power, and fears about possible future threats to the stability of the new member states to be admitted in enlargement, the Treaty of Nice added a preventive mechanism to sanctions against a Member State that was created by the Amsterdam Treaty.

The Irish government, having obtained the Seville Declaration on Ireland's policy of military neutrality from the European Council, decided to have another referendum on the Treaty of Nice on Saturday, 19 October 2002.

Two significant qualifications were included in the second proposed amendment, one requiring the consent of the Dáil for enhanced cooperation under the treaty, and another preventing Ireland from joining any EU common defence policy.

Prominent civil society campaigns on the Yes side included Fianna Fáil, Fine Gael, the Labour Party, the Progressive Democrats, the Irish Alliance for Europe led by Professor Brigid Laffan and Adrian Langan, and Ireland for Europe led by Ciarán Toland.

They claim that, consequently, the treaty was vitally important for the integration and future progress of these former Eastern Bloc countries.

Many people who were in favour of greater scope and power of the EU project felt that it did not go far enough and that it would in any case be superseded by future treaties.

[citation needed] Opponents pointed out that leading pro-treaty politicians had admitted that if referendums had been held in countries other than Ireland, it would probably have been defeated there as well.

The EU inherited many of its present responsibilities from the European Communities (EC), which were founded in the 1950s in the spirit of the Schuman Declaration.