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. Estonia is also a member of the Organization for Security and Cooperation in Europe (OSCE), Organisation for Economic Co-operation and Development (OECD), Council of the Baltic Sea States (CBSS) and the Nordic Investment Bank (NIB). As an OSCE participating State, Estonia's international commitments are subject to monitoring under the mandate of the U.S. Helsinki Commission. Estonia has also signed the Kyoto Protocol.

What date did Estonia join the League of Nations?