The North Atlantic Treaty was signed on 4 April 49.
The original signers were
Belgium, Canada, Denmark, France, Iceland, Italy, Luxembourg, Netherlands, Norway, Portugal, United Kingdom, and the United States.
The treaty is one of the shortest treaties signed.
http://www.nato.int/docu/basictxt/treaty.htm
While not perfect (what is?), NATO seemed to do its job for sixty years.
Trivia question: When was the first time the "mutual self-defense" clause of NATO was used?