After the World War I, Poland regained its independency and access to the Baltic Sea. City of Danzig was granted a status of Free City, with most German population living there. Polish government was trying to set a new location for a seaport. Small village Gdynia in just couple of years became one of the most important cities in Poland and in the Baltic Sea basin.
Length of border with sea was just nearly 150 km comparing to 770 km nowadays.