The Free City of Danzig (Freie Stadt Danzig; Wolne Miasto Gdańsk) was a city-state under the protection of the League of Nations between 1920 and 1939, consisting of the Baltic Sea port of Danzig (now Gdańsk, Poland) and nearly 200 other small localities in the surrounding areas. The polity was created on 15 November 1920 in accordance with the terms of Article 100 (Section XI of Part III) of the 1919 Treaty of Versailles after the end of World War I. In line with the treaty provisions, the entity was established under the oversight of the League of Nations. Although predominantly German-populated, the territory was bound by the imposed union with Poland covering foreign policy, defence, customs, railways and post, while remaining distinct from both the post-war German Republic and the newly independent Polish Republic. In addition, Poland was given certain rights pertaining to port facilities in the city.
In the 1920 Constituent Assembly election, the Polish Party received over 6% of the vote, but its percentage of votes later declined to about 3%. A large number of Danzig Poles voted for the Catholic Centre Party instead. In 1921, Poland began to develop the city of Gdynia, then a midsized fishing town. This completely new port north of Danzig was established on territory awarded in 1919, the so-called Polish Corridor. By 1933, the commerce passing through Gdynia exceeded that of Danzig. By 1936, the city's senate had a majority of local Nazis, and agitation to rejoin Germany was stepped up. Many Jews fled from German antisemitism, persecution, and oppression. In 1938, the Free City's population of 410,000 was 98% German, 1% Polish and 1% other. Historical estimates put the proportion of Danzig Poles between at between 10 and 13 %. In the 1920s and 1930s, the Polish population increased, and the proportion of Poles in the population of the Free City was estimated at around 20 % in 1939.
After the German invasion of Poland in 1939, the Nazis abolished the Free City and incorporated the area into the newly formed Reichsgau of Danzig-West Prussia.