According to Wikipedia, Mrągowo (Sensburg in Polish) is a resort town in the Warmian-Masurian Voivodeship of northeastern Poland.
Following World War I, as a condition of the Treaty of Versailles, the League of Nations held the East Prussian plebiscite on 11 July 1920 to determine if the people in the southern districts of East Prussia wanted to remain within the Free State of Prussia and Germany or to join the Second Polish Republic. Before the vote German nationalists engaged in brutal excesses, which remained unhindered by meagre presence of Allied forces.The plebiscite resulted in 3,660 votes for Germany and none for Poland.
https://en.wikipedia.org/wiki/Mr%C4%85gowo
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Wikidata: Q745085