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")
National Defense units were composed of volunteers, some reservists and unemployed. They were equipped with second-grade weaponry, and their goal was to support defensive operations of the regular Polish Army. National Defense was created by an order of the Polish Ministry of Military Affairs on 5 December 1936; the brigades begun to be formed in 1937.
During the German invasion of Poland in 1939, National Defense mobilized 83 battalions (approximately 1,600 officers and 50,000 soldiers).