 Following the First Partition of Poland in 1772, the town, as "Lessen", was annexed by King Frederick II of Prussia and made part of the German Kingdom of Prussia.In 1871, during the unification of Germany, it became part of the Prussian-led German Empire and in 1920, in accordance with the Treaty of Versailles, Łasin became part of the Polish Republic, after it regained independence in 1918.In interwar Poland, the mayor of Łasin was Stefan Tomczyński, a Polish activist and efficient administrator, who was previously harassed by Prussians for pro-Polish activity in the Prussian Partition.