Pursuant to Article 10 of the Washington Treaty, NATO nations may, "by unanimous agreement, invite any other European State in a position to further the principles of [the] Treaty and to contribute to the security of the North Atlantic area to accede to [the] Treaty". In addition, aspirant countries must meet certain political criteria, including a democratic political system, a free-market economy, fair treatment of minorities, and the will to peacefully settle disputes. Depending on a country’s situation, accession to NATO will require more or less substantial reform efforts on the part of the aspirant.
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For more information on NATO enlargement and on the Membership Action Plan: