NATO has an open door policy on enlargement. Any European country in a position to further the principles of the North Atlantic Treaty and contribute to security in the Euro-Atlantic area can become a member of the Alliance, when invited to do so by the existing member countries. The fifth round of NATO enlargement may not be the last. At present, three countries - Albania, Croatia and the former Yugoslav Republic of Macedonia - are members of NATO’s Membership Action Plan (MAP), designed to assist aspiring partner countries to meet NATO standards and prepare for possible future membership. Aspirant countries are expected to participate in the Membership Action Plan to prepare for potential membership and demonstrate their ability to meet the obligations and commitments of possible future membership. They must then be officially invited by NATO to begin accession talks with the Alliance. Invitations to join the Alliance are issued by the North Atlantic Council, NATO’s principal decision-making body. Relations with partner and aspirant countries are maintained by NATO’s international staff as well as committees, subordinate to the Council.
“The accession talks are a series of meetings between a team of NATO experts and individual invitees to discuss and formally confirm their interest, willingness and ability to meet the political, legal and military obligations and commitments of NATO membership. They cover the formal obligations of NATO membership. NATO experts and invitees also discuss specific issues and reforms. The final product of these discussions will be a timetable to be submitted by each invitee for the completion of these reforms.
Letters of intent: Foreign ministers of the invited countries send letters of intent to NATO confirming their interest, willingness and ability to join the Alliance, as well as timetables for completion of reforms.
Accession protocols: NATO prepares accession protocols to the North Atlantic Treaty for each one of the invited countries. The protocols are formal, legal documents that pave the way for invited countries to accede to the North Atlantic Treaty, once they have been signed and ratified by the existing member countries. …..
Ratification of accession protocols: After signing the accession protocols, NATO member countries must ratify, accept or approve them, in accordance with national requirements and procedures, which vary from country to country. Once the ratification process is complete, the prospective new members are formally invited to become parties to the North Atlantic Treaty.
Invitees become NATO members: Ratification procedures relating to the accession process also have to be implemented in the invited countries in accordance with varying national constitutional arrangements. Once the procedures have been completed, the invited countries will deposit the instruments of accession with the Government of the United States, as repository country, in accordance with Article 14 of the North Atlantic Treaty of 1949.” [5]

Concrete example of the newest member countries (March 2004):
1) December 2002 - March 2003: Accession talks
2) January 2003 - March 2003: Invitees send letters of intent to NATO
3) March 2003: Accession protocols are signed
4) 2003-2004: Accession protocols are ratified by NATO countries
5) By May 2004: Invitees become NATO members

[5] - The Prague Summit and NATO’s Transformation, p. 24 - 25
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