In the 19th century, a Russian ethnographer found a Slavic ethnic group populating Eastern parts of Western Pomerania. He called them Slovincy, despite the fact that their language was so closely related to Kashubian that it could be regarded as its dialect. Slovincians regarded themselves as Kashubs-Lutherans and their language as Kashubian.
The same process happened for Kashubians. Nevertheless, the fact that Kashubians were Roman Catholics and Kulturkampf set them against government policies, and also due to support from the growing Polish nationalism, Kashubians survived until the Treaty of Versailles put them under Polish government.
At the same time, the Slovincy were left in the borders of Germany, where they shared the faith of Germans, including taking part in Nazism.
Areas populated by the Slovincy became part of Poland in 1945. The fact that the Slovincy took part in WWII on the German side prevented the new Polish government from regarding them as Slavic brothers. The last speakers lived in parishes of Schmolsin and Garde in Pomerania before they left for Germany after 1945, so the Slovincian language became extinct around 1950.