1920 Schleswig plebiscites
The Schleswig plebiscites were two plebiscites, organized according to section XII, articles 109 to 114 of the Treaty of Versailles of 28 June 1919, in order to determine the future border between Denmark and Germany through the former Duchy of Schleswig. The process was monitored by a commission with representatives from France, the United Kingdom, Norway and Sweden.
Source: Wikipedia — 1920 Schleswig plebiscites (CC BY-SA 4.0)