Joining NATO is governed by Article 10 of the North Atlantic Treaty, which states:
The Parties may, by unanimous agreement, invite any other European State in a position to further the principles of this Treaty and to contribute to the security of the North Atlantic area to accede to this Treaty. Any State so invited may become a Party to the Treaty by depositing its instrument of accession with the Government of the United States of America. The Government of the United States of America will inform each of the Parties of the deposit of each such instrument of accession.
Now, it is clear from the text of Article 10, that an invitation to join NATO requires unanimous agreement. The text of Article 10 is silent on how that unanimous agreement is expressed, however. In practice, ever since the accession of Greece and Turkey in 1951, that unanimous agreement has been expressed through a formal protocol to the treaty, requiring ratification by each member state (through their respective national procedures, which usually means a vote of their national legislature, but each country gets to decide the details of that procedure). How and why did NATO decide that unanimous agreement had to be expressed in this way–as opposed to merely a unanimous decision of the North Atlantic Council (the NATO governing body established by Article 9, on which each member state has a seat)? The wording of the article requires unanimity, but it never says that unanimity must be expressed through a treaty-level protocol.
Although NATO and the EU are very different organisations, there are certain similarities between their enlargement processes – both require unanimous agreement for any enlargement, and both require enlargement to be done through a formal Protocol to their founding treaties. However, unlike Article 10 of the North Atlantic Treaty, the equivalent article of the Treaty on European Union, Article 49, says:
Any European State which respects the values referred to in Article 2 and is committed to promoting them may apply to become a member of the Union. The European Parliament and national Parliaments shall be notified of this application. The applicant State shall address its application to the Council, which shall act unanimously after consulting the Commission and after receiving the consent of the European Parliament, which shall act by a majority of its component members. The conditions of eligibility agreed upon by the European Council shall be taken into account.
The conditions of admission and the adjustments to the Treaties on which the Union is founded, which such admission entails, shall be the subject of an agreement between the Member States and the applicant State. This agreement shall be submitted for ratification by all the contracting States in accordance with their respective constitutional requirements.
So the EU's founding treaty makes explicit the requirement for both unanimity and a treaty protocol; NATO's founding treaty appears to only explicitly demand unanimity, yet somehow it still requires a treaty protocol in practice. Why?