The books are closed. Not accepting any new members
this territory is moderated
Not really. But new potential NATO members are limited to Europe.
See Article 10 of the Treaty:
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.
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what is "instrument of accession"?
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Formal document, where state asks that it wants to join NATO. Name likely comes from partition of India in 1947 (https://en.wikipedia.org/wiki/Instrument_of_Accession).
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US Senate must approve with 2/3 super majority
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