Since the 1991 Rome summit , when the delegations of its member states officially offered cooperation with Europe 's newly democratic states , NATO has addressed and further defined the expectations and procedure for adding new members . The 1994 Brussels Declaration reaffirmed the principles in Article 10 and led to the " Study on NATO Enlargement " . Published in September 1995 , the study outlined the " how and why " of possible enlargement in Europe , highlighting three principles from the 1949 treaty for members to have : " democracy , individual liberty , and rule of law "
1991 Rome summit
1991 Rome summit
Enlargement of NATO