Wizna was a town (now a village) that in 1939 was an important defense point due to its location near the bridges and fords on the Narew river, between swamplands and forests on the banks of the rivers Narew and its tributary Biebrza. The importance of the area was well-known for decades and in early 20th century Russian government (the area belonged to Russia back then) ordered a construction of modern fortifications that proved to be crucial in defending against German assault in the Great War. These somewhat obsolete (from the perspective of 1930s) fortifications were undergoing modernization when the war broke out. New bunkers were modern and proved to be a good defense against infantry, they were too sparsely located (the construction of the new fortification line was not finished until 1st of September 1939) and lacked anti-tank artillery, making them sub-par against an armoured attack. In the Battle of Wizna, this particular element of the Polish defense line was manned by about 360 soldiers, including officers.

It should be stressed, that the commonly referenced 40:1 ratio actually closer to 58:1) reflected only the comparison of the troops dislocated in the area, not the forces taking part in actual battle. In reality, Polish soldiers were defending against much smaller opposition, with the majority of the German forces stranded on the opposite banks of the Narew and Biebrza rivers that are quite deep in that area. This meant, of course, that they were still heavily outnumbered and outgunned, but the actual number of attackers was much smaller than the overall number of German soldiers in the area. Last but not least, what is called 'Battle of Wizna' was actually fought on the Ciełczyn-Łynki fortified line with the command bunker on Strękowa Góra, as Wizna itself has been captured on 7th of September by the reconnaissance detachments of the 10th Panzer Division.

The main battle took place on 9th and early 10th of September and gen, Heinz Guderian, present at the site, used the first attacks to perform reconnaissance by combat, trying to evaluate actual defensive capabilities of the Polish forces. This led to a limited and rather prolonged combat, during which Guderian was able to spot the main deficiencies of the defensive positions and use it to his advantage on the following day. Namely, he noticed that the main anti-tank armament of the defenders amounted to the field artillery (six 75 mm 1902/26 guns wih roughly 200 rounds between them) and few (later revealed to be only two) anti-tank rifles with a very limited supply of ammunition (they were assigned a single supply unit each, i.e. 20 rounds in total). This is why on 10th of September, assuming that defenders depleted or heavily strained their ammo reserves, he sent tanks to isolate points of defense, making them much susceptible to be overrun by the advancing infantry. Seeing that the defeat is inevitable, Cpt. Władysław Raginis ordered remaining defenders to surrender and then committed suicide.

So, why the delay? According to the accounts of both German soldiers, including Guderian himself, Polish defenders put an extremely efficient and intensive resistance, leading the German commanders to an assumption that the forces are far more numerous and far better supplied than they really were, and thus making them reluctant to risk serious casualties in an all-out assault, especially that they were forced to attack from the front as Polish defenders in the area demolished nearby bridges on the Narew and the arrrival of pontoon bridges was delayed due to heavy traffic, while the number of pontoons available on site was too low to be of use (it is not clear, whether Germans had improper data, miscalculated the requirements or the river itself flooded the neighbouring area shortly prior to the German invasion). But when the ammunition god depleted and significant number of defenders suffered wounds, Guderian saw Polish bluff and realizing that he is able to successfully push through without much risk, he did exactly that.

This is not to diminish the combat prowess of the defenders. The position was impossible to defend even against much smaller attacking forces, mainly due to the small amount of ammunition available. The sacrifice of the soldiers and successful bluff made by them stalled the army for two days, giving the other units withdrawing towards the second line of defense on he Vistula river to form a new army or leave the country through the Polish-Romanian border.