In the early years of the Cold War, the U.S., Canada and 10 Western European nations forged a collective defense pact to protect the continent against Soviet expansion. The North Atlantic Treaty Organization (NATO) members pledged to respond as one if any member were attacked.

Today there are 28 NATO members, and the alliance formally tendered an invitation May 19 to Montenegro to become the 29th. It will be the first expansion since Albania and Croatia joined in 2009.

In all, NATO admitted a dozen new members between 1999 and 2009, including 10 former Warsaw Pact countries and Slovenia and Croatia, both once parts of Yugoslavia.

Montenegro is a small Balkan country on the Adriatic coast with a population of 620,000. It provided support for the NATO-led operation in Afghanistan from 2010 to 2014.

NATO Secretary-General Jens Stoltenberg said membership “will bring more stability and security to the region and therefore [promote] prosperity.”

Indeed, joining NATO has paid dividends for members that go well beyond increased security by spurring growth of their free-market economies.

Graphs showing increase of gross domestic product (World Bank, State Dept.)
Gross domestic product growth for the dozen former Eastern bloc countries that have joined NATO since 1999. (World Bank, State Dept.)

Under the Brussels-based alliance’s “open door policy,” membership is open to any “European state in a position to further the principles of [the 1949 North Atlantic Treaty] and to contribute to the security of the North Atlantic area.”

Countries must convince the Allies not only that they are committed to collective defense but also that they meet its political, military and economic standards:

  • A functioning democratic political system based on a market economy.
  • Fair treatment of minority populations.
  • Commitment to resolve conflicts peacefully.
  • Ability and willingness to contribute militarily to NATO operations.
  • Commitment to democratic civil-military relations and institutions.

NATO says three other nations — Bosnia and Herzegovina, Georgia, and the former Yugoslav Republic of Macedonia — aspire to membership.

The decision to admit a new member must be unanimous. Each ally makes its own judgment on whether a country meets NATO’s standards. The ratification procedures vary. The U.S. requires a vote of approval by a two-thirds Senate majority. The United Kingdom does not submit the question to a vote in Parliament.

Members and year of admission: Belgium, Canada, Denmark, France, Iceland, Italy, Luxembourg, Netherlands, Norway, Portugal, United Kingdom and United States (1949); Greece and Turkey (1952); Germany (1955); Spain (1982); Czech Republic, Hungary and Poland (1999); Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia, and Slovenia (2004); Albania and Croatia (2009).