During World War I, the Battle of Tannenberg and the First and Second Battle of the Masurian Lakes between Imperial Germany and the Russian Empire took place within the borders of Masuria in 1914. After the war, the League of Nations held the East Prussian plebiscite on 11 July 1920 to determine if the people of the southern districts of East Prussia wanted to remain within East Prussia or to join the Second Polish Republic. The German side terrorised the local population before the plebiscite using violence, Polish organisations and activists were harassed by German militias, and those actions included attacks and some supposed murders of Polish activists; Masurs who supported voting for Poland were singled out and subjected to terror and repressions.