Twelve countries created NATO by signing the North Atlantic Treaty, and Article 10 keeps the door open to new members

Background

NATO was created by 12 countries from Europe and North America on 4 April 1949.
The Foreign Ministers of those twelve signed the North Atlantic Treaty, also known as the Washington Treaty, at the Departmental Auditorium in Washington, D.C.
The founding member countries were Belgium, Canada, Denmark, France, Iceland, Italy, Luxembourg, the Netherlands, Norway, Portugal, the United Kingdom and the United States.
Since then 20 more countries have joined through 10 rounds of enlargement, and at present NATO has 32 member countries.
Article 10 of the North Atlantic Treaty sets out how countries can join, and states that membership is open to any European State in a position to further the principles of the Treaty and to contribute to the security of the North Atlantic area.
Any decision to invite a country to join is taken by the North Atlantic Council, NATO's principal political decision-making body, on the basis of consensus among all Allies.
Greece and Turkiye were the first countries to accede after the founding twelve, three years after the signing of the Washington Treaty.
When Iceland signed the Treaty it did not have, and still does not have, armed forces, though it keeps a Coast Guard, national police forces and an air defence system.