These were the countries which were considered to be sufficiently prepared for Alliance membership and which fulfilled the conditions that had been established. Alliance leaders reaffirmed that NATO would remain open to other new members.

- September and November 1997. Accession talks were held with each of the three invited countries. At the end of the process, the three countries sent letters of intent confirming commitments undertaken during the talks. - 16 December 1997. NATO Foreign Ministers signed Protocols to the North Atlantic Treaty on the accession of the three countries.

- During 1998, Allied countries ratified the Protocols of Accession according to their national procedures.

- 12 March 1999. After completion of their own national legislative procedures, the Foreign Ministers of the Czech Republic, Hungary and Poland deposited the official documents relating to accession to the North Atlantic Treaty (known as the "instruments of accession") in a ceremony in Independence, Missouri, in the United States. The completion of this requirement marked their formal entry into the Alliance.

- 16 March 1999. The national flags of the three new member states were raised at a ceremony at NATO headquarters, Brussels. At the Madrid Summit Meeting in 1997, NATO Heads of State and Government encouraged other candidate states to continue to work towards eventual membership by following the guidelines laid out in the Study on NATO Enlargement and developing bilateral cooperation with NATO through the Partnership for Peace programme. They reaffirmed the Alliance's commitment to the “open door” policy in which all European countries meeting the conditions of Article 10 and the guidelines of the study could be considered for eventual membership. This policy was reaffirmed at the Washington Summit in 1999 and a Membership Action Plan (MAP) was launched to assist aspiring countries to prepare for membership.

The MAP is designed to provide a programme of activities from which countries may select those they consider of most value to help them in their preparations for possible future membership. The Plan calls for aspirants to submit individual annual national programmes on their preparations. It also provides mechanisms for feedback and advice on progress made by them in implementing these programmes.