The Suwałki Agreement, Treaty of Suvalkai, or Suwalki Treaty (, ) was an agreement signed in the town of Suwałki between Poland and Lithuania on October 7, 1920. It was registered in the "League of Nations Treaty Series" on January 19, 1922. Both countries had re-established their independence in the aftermath of World War I and did not have well-defined borders. They waged the Polish–Lithuanian War over territorial disputes in the Suwałki and Vilnius Regions. At the end of September 1920, Polish forces defeated the Soviets at the Battle of the Niemen River, thus militarily securing the Suwałki Region and opening the possibility of an assault on the city of Vilnius (Wilno). Polish Chief of State, Józef Piłsudski, had planned to take over the city since mid-September in a false flag operation known as Żeligowski's Mutiny.