National Defense (Polish: Obrona Narodowa, ON) was a volunteer military formation of the Second Polish Republic. Its units (brigades and demi-brigades, divided into battalions) were subordinated to various Polish corps and armies. National Defense is most commonly referred through its units (brigades - sing. Brygada Obrony Narodowej (Brygada ON) or battalions - sing. Batalion Obrony Narodowej (Batalion ON); the units were also named by the geographical region they were related to, and if more than one was formed, by Roman numerals, ex. Batalion ON "Lwów II")