The first battle between the forces of Švitrigaila and Sigismund Kęstutaitis is fought near the town of Oszmiana (Ashmyany), launching the most active phase of the Lithuanian Civil War.

Švitrigaila was the Grand Duke of Lithuania from 1430 to 1432. He spent most of his life in largely unsuccessful dynastic struggles against his cousins Vytautas and Sigismund Kęstutaitis.

Sigismund Kęstutaitis was Grand Duke of Lithuania from 1432 to 1440. Sigismund was his baptismal name, while his pagan Lithuanian birth name is unknown. He was the son of Grand Duke Kęstutis and his wife Birutė.

Ashmyany or Oshmyany is a city in Grodno Region, Belarus. It is located 50 kilometres (31 mi) from Vilnius in Lithuania, and serves as the administrative center of Ashmyany District. The river Ashmyanka passes through the city. As of 2025, it has a population of 16,804.

The Lithuanian Civil War of 1432–1438 was a war of succession to the throne of the Grand Duchy of Lithuania, after Vytautas the Great died in 1430 without leaving an heir. The war was fought on the one side by Švitrigaila, allied with the Teutonic Knights, and on the other by Sigismund Kęstutaitis, backed by the Kingdom of Poland. The war threatened to sever the Union of Krewo, the personal union between Poland and Lithuania. Švitrigaila's alliance with the Grand Master of the Teutonic Order, Paul von Rusdorf, launched the Polish–Teutonic War (1431–1435) but failed to secure victory for Švitrigaila.

December 9 is the 343rd day of the year in the Gregorian calendar; 22 days remain until the end of the year.