The Amsterdam Treaty, which came into effect on May 1, 1999, was supposed to prepare the European Union for the enlargement, but it did not succeed. Therefore, the member states at the European Council in Helsinki at the end of 1999 agreed that a treaty addressing open institutional issues or the so-called Amsterdam leftovers must be adopted before further enlargement.

This agreement was fulfilled with the Nice Treaty, signed on February 26, 2001, and it came into effect on February 1, 2003. This means that, according to the decisions of the European Council in 1999, the treaty came into force with a one-month delay. Complications arose when the Irish initially rejected the treaty in a referendum. In a second referendum, the Irish supported the treaty, and thus, the EU expanded to 25 members on May 1, 2004.

The European Union is based on the rule of law, meaning that every EU measure is based on treaties ratified by all EU member states through a democratic process. If the Nice Treaty had not been ratified by all member states, the doors to the EU would have remained closed for the ten countries on May 1, 2004. The same principle of the rule of law applies to accession treaties for future EU enlargement. In democratic processes, every accession treaty will have to be ratified by all EU member states. Therefore, it is crucial to address all concerns and seek common solutions in accession negotiations.