Original: Estonia was a member of the League of Nations from 22 September 1921 , has been a member of the United Nations since 17 September 1991 , and of NATO since 29 March 2004 , as well as the European Union since 1 May 2004 .
Simplified: Estonia became a member of the North Atlantic Treaty Organization ( NATO ) on 29 March 2004 . It joined the European Union on 1 May 2004 .