The 18th Infantry Division (18. Dywizja Piechoty) was a unit of the Polish Army during the interbellum period, which took part in the Polish September Campaign. Stationed in Łomża and commanded in 1939 by Colonel Stefan Kossecki, it was part of the Narew Independent Operational Group.
It was formed in 1919 from the units of the Blue Army.
Between 1919 and 1939, Łomża and neighboring towns were located very close to the border of Poland and German province of East Prussia. Defense of this area was regarded as crucial in any future conflict, so 18th Division was regarded as an elite unit. More than 50% of its soldiers were local conscripts from northern Mazovia and Podlasie, whose patriotism was highly appreciated.
The Division assembled on August 30, 1939, and the next day it occupied defense positions along the Narew river. Until September 3, it had a limited contact with the Wehrmacht. It was then ordered to take positions previously held by the Polish 41st Infantry Division. On September 7, it left Ostrołęka and was ordered to halt the advance of German 21st Infantry Division near Nowogrod. The efforts of Polish soldiers were mixed, but on the night of Sept 9-10, it attacked a German armored column, destroying several vehicles and tanks.
During the next days the Division was engaged in heavy fights with units of General Heinz Guderian's XIX Panzer Corps. After several bloody skirmishes, the Germans severely wounded Colonel Kossecki. 20 volunteers, who wanted to take his body from the battleground, were killed by a German machine gun. Kossecki himself survived, but was captured by the Soviets and killed by them probably in 1940 (see: Katyn Massacre).
In the night of September 13–14 the Germans massacred 200 Polish POWs, gathered in the courtyard in the barracks in Zambrów. The Division ceased to exist.