Gdańsk became a large and rich Polish seaport city during the reign of king Kazimierz IV Jagiełłoński (Casimir the Jagiellonian). The Polish Golden Age (the 16th and the 17th centuries) brought trade and culture to Gdańsk. The city's economy slowly declined in the 18th century, which ended in the Partitions of Poland in 1772-1793. Gdańsk was annexed by Prussia to become the capital of West Prussia in 1814 and became part of the German Empire in 1871. The majority of the people of Danzig spoke two languages: German and Polish.
After World War I, Poland recovered its independence. The Poles hoped to restore their city with its free access to the sea, as they had been promised by the allies. With the Treaty of Versailles, the Hanseatic city was separated from the German Empire as an independent city on 10 January 1920. On 15 November 1920, allied and associated powers protect Danzig under the League of Nations, calling it "Freie Stadt Danzig" (Free City of Danzig). In practice, the city was predominantly German and the Germans claimed full control. Due to obstruction and restriction of Polish trade and settlement by the German authorities, the Polish government invested in the building of another seaport, just north of Gdańsk, called Gdynia.
In the harbour of Danzig, a Polish post office operated, using Polish stamps overprinted "Port Gdańsk". The office operated from 5 January 1925 until 1 September 1939.
The tension between Germany and Poland over the control of the Free City culminated in a full scale war. On 1 September 1939, Germany invaded Poland at Westerplatte (Gdańsk), starting World War II. Gdańsk was annexed by the nazis.
On 30 March 1945, most of Danzig was reduced to a mass of rubble when the Germans defended the city from Polish and Soviet troops. At the Yalta Conference and the Potsdam Conference, Danzig ceded to full Polish sovereignty. The city was rebuilt in the 1950s and 1960s to become a major port and industrial centre of the People's Republic of Poland, once again known as Gdańsk. Only from 1990, Gdańsk belongs to Poland (now a republic) according to international law.
Danzig had used Prussian stamps, Reichspost issues and when Danzig was given the status of Free city, the first issues were the Germania series overprinted Danzig, followed by stamps that gave the Free City of Danzig its own distinct identity. With the start of World War II, Danzig was incorporated into the Reich. After World War II, Danzig was occupied by the Soviets, the city was bombed to pieces and made a part of Poland.
The Free City of Danzig became a member of the Universal Postal Union on 1 October 1920.
Free City of Danzig
Danzig online (by Wojciech Gruszczyński)
WHKMLA: History of Danzig