Polish–Lithuanian War: The Polish Army launches the Vilna offensive to capture Vilnius in modern Lithuania.

The Polish–Lithuanian War was an undeclared war fought in the aftermath of World War I between newly independent Lithuania and Poland, with fighting mainly in the Vilnius and Suwałki regions, which was part of the Lithuanian Wars of Independence and lasted from May 1919 to 29 November 1920. From the spring of 1920 onward, the conflict happened alongside the wider Polish–Soviet War and was affected by its progress. It was subject to unsuccessful international mediation at the Conference of Ambassadors and the League of Nations.