In 1910, Lyck had more than 13,000 inhabitants. Mateusz Siuchniński gives the percentage of Poles in 1900 as 35.7% but warns that the numbers come from lowered German estimates. Many citizens fled during World War I, when Imperial Russian troops attacked the region, but returned after the battles of Tannenberg and the Masurian Lakes. English and Italian troops were deployed in the town after the Treaty of Versailles to supervise the East Prussian plebiscite, which resulted in 8,339 votes for Germany and 8 for Poland.