The 'Battle of Wizna' was fought between German and Polish forces in the early stages of the German 'Weiss' (i) invasion of Poland (7/10 September 1939).

In this battle, a force of between 350 and 720 Poles defended a fortified line against more than 40,000 Germans. Although defeat was inevitable, the Polish defence stalled the attacking forces for three days and delayed the encirclement of Generał brygady Czesław Młot-Fijałkowski’s Operational Group 'Narew'. Eventually the German armour broke through the Polish line and German engineers eliminated all the bunkers, of which the last surrendered at about 12.00 on 10 September.

Because the battle took the form of a small force holding fortified territory against a vastly larger and better armed invasion before being annihilated, the 'Battle of Wizna' is sometimes known as the 'Polish Thermopylai'.

Before the start of 'Weiss' (i), the area of the village of Wizna was readied by the Poles as a fortified defence line to shield the Polish positions farther to the south and to guard the crossings of the Narew and Biebrza rivers. The 5.6-mile (9-km) line of Polish defences extended between the villages of Kołodzieje and Grądy-Woniecko, with Wizna in its centre. The line was sited some 22 miles (35 km) from Poland’s border with East Prussia, along the elevated banks of the Narew and Biebrza rivers. Units defending the line were subordinate to the Operational Group 'Narew' shielding Łomża and providing the defence of the northern approach to Warsaw. The Wizna fortified area was one of the most important defensive positions of northern Poland as it covered both the river crossings as well as the roads linking Łomża with Białystok and that to Brześć Litewski in the rear of the Polish forces.

Construction of the defences started in June 1939, only two months before the German invasion. The location was selected with great care: most of the concrete bunkers were built on hills overlooking the swampy Narew river valley. The bunkers could be approached and attacked only through direct assault through the swamps or by attack along the causeway leading from the bridge in Wizna. Before 1 September, only 16 of the 60 planed bunkers had been built: six of the bunkers were of heavy concrete and topped with a steel cupola armed with machine guns and anti-tank guns; two of the bunkers were of lighter concrete construction and armed only with machine guns; and the remaining eight bunkers were extemporised machine gun pillboxes, protected mostly by sandbags and earthworks. Four additional heavy bunkers were under construction at the time the Germans launched their invasion. The defence of the area also included trenches, anti-tank and anti-personnel obstacles, barbed wire entanglements and mines. The Poiles also planned to break the dams on the Biebrza and Narew rivers in order to flood the area, but the summer of 1939 was one of the driest in Polish history and the water level was too low to make the plan feasible.

Although not all the bunkers were ready by the beginning of the war, the Polish defence lines were well-prepared. The walls of an average bunker, 4.9 ft (1.5 m) thick and reinforced with steel places 8 in (200 mm) thick, and could withstand a direct hit from even the heaviest guns available to the Germans at the time. The bunkers were sited on hills which provided good visibility over all possible axes of advance.

The Polish defensive line was initially manned by a single battalion of the 71^st Regiment, commanded by Major Jakub Fober. Shortly before the outbreak of hostilities, however, the defences were supplemented with a machine gun company from Osowiec Fortress under Kapitan Władysław Raginis, as well as numerous smaller detachments from several other units. On 2 September, the 3/71^st Regiment departed for Osowiec, and Fober was succeeded in command by Raginis. Altogether, the Polish defensive position was manned by 720 men in the form of 20 officers and 700 other ranks, but some sources claims that the garrison was only 360 men.

Although the Polish units were composed almost entirely of conscripts mobilised in August rather than professional soldiers, their morale was very high. The defenders were the 8^th Company of the 135^th Regiment, the 3^rd Heavy Machine Gun Company of the Osowiec fortress battalion, one battery of artillery, the 136^th Engineer Company, one engineer platoon and one field artillery platoon of the 71^st Regiment and one mounted reconnaissance platoon of the 135^th Regiment in defences centred on 12 bunkers with six 75-mm (2.95-in) pieces of artillery, 24 heavy machine guns, 18 light machine guns and two anti-tank rifles, for which there were only 20 rounds.

The opposing German forces were elements of General Georg von Küchler’s 3^rd Army within Generaloberst Fedor von Bock’s Heeresgruppe 'Nord', which was to advance into north-eastern Poland from East Prussia, although its eastern spearhead was General Heinz Guderians XIX Corps (mot.), a formation of General Günther von Kluge’s 4^th Army that had already surged from Pomerania across the 'Polish corridor' to reach East Prussia and take up position on the German forces' eastern flank. The XIX Corps (mot.) comprised Generalleutnant Leo Freiherr Geyr von Schweppenburg’s 3^rd Panzerdivision, Generalleutnant Mauritz von Wiktorin’s 20^th Division (mot.), Generalleutnant Ferdinand Schaal’s 10^th Panzerdivision and Oberst Otto-Ernst Offenbacher’s Brigade 'Lötzen'. The German strength was 42,000 men, 350 tanks, 108 howitzers, 58 field guns, 195 anti-tank guns, 108 mortars, 188 grenade launchers, 288 heavy machine guns and 689 machine guns

On 1 September the 3^rd Army began its advance to the south in the direction of Warsaw, directly through the positions of Operational Group 'Narew', and the XIX Corps (mot.) started to move farther to the east in Poland with an advance to the south-east in the direction of Brest-Litovsk. It was on the following day that Raginis was named as the commander of the Wizna area. As his command post Reginis selected the GG-126 bunker near the village of Góra Strękowa. This bunker was located on a hill in the exact centre of the Polish defensive line.

After initial clashes at the border, during the night of 3/4 September the Cavalry Brigade 'Podlaska' was ordered to withdraw, and on 5 September left the area to march toward Mały Płock, where it was to cross the Narew river. On 3 September Polish positions were spotted by German aircraft and strafed with machine gun fire.

On 7 September, the reconnaissance units of the 10^th Panzerdivision took the village of Wizna. Polish mounted reconnaissance groups abandoned the village after a short fight and retreated to the southern bank of the Narew river. As German tanks tried to cross it, the bridge was blown by Polish engineers. After dark, German infantry patrols crossed the river and advanced towards Giełczyn, but were repelled with heavy casualties.

On 8 September, Guderian was ordered to advance through Wizna toward Brześć. By a time early in the morning of 9 September his units had reached the Wizna area and linked with the 10^th Panzerdivision and the Brigade 'Lötzen' already in the area, giving him a numerical advantage of some 60/1 over the Polish defenders.

Early in this morning, German aircraft dropped leaflets urging the Poles to surrender, claiming that most of Poland was already in German hands, and stating that further resistance was futile. In order to strengthen his men’s morale, Raginis and the local artillery commander swore that they would not leave their post alive and that resistance would continue. Soon after this moment, a German artillery barrage and aerial bombardment started. The Polish artillery was much weaker than that of the Germans and was soon forced to retreat toward Białystok. After preparation, the Germans attacked the northern flank of the Polish force. Two platoons defending several bunkers located to the north of the Narew river were attacked from three sides by German tanks and infantry. Initially the Germans' infantry losses were high, but after receiving heavy artillery fire, the commander of the Giełczyn area, Porucznik Kiewlicz, was ordered to burn the wooden bridge over the Narew river and withdraw to Białystok. The remnants of his force broke through the German encirclement and reached Białystok, where they joined the IX Corps of Generał brygady Franciszek Kleeberg.

At the same time, an assault on the southern part of Polish fortifications degenerated into a stalemate. The Polish bunkers lacked adequate anti-tank armament, but were able to rake the German infantry with machine gun fire. However, at 18.00 the Polish infantry was forced to abandon its trenches and field fortifications, and fell back into the bunkers. The German tanks could finally penetrate the Polish line and advance toward Tykocin and Zambrów. However, the German infantry was still under heavy fire and pinned down in the swampy fields in front of the Polish bunkers.

Although Raginis was subordinate to Podpułkownik Tadeusz Tabaczyński, commander of the Osowiec fortified area located some 18.6 miles (30 km) to the north, he could expect no reinforcement. On 8 September, Marszałek Polski Edward Rydz-Śmigły, the Polish commander-in-chief, ordered the 135^th Regiment, which constituted the reserves of both Osowiec and Wizna, to be withdrawn to Warsaw. The order was soon rescinded and the unit returned to Osowiec, but it was already too late to help the isolated Poles at Wizna.

Heavy fighting for each of the now isolated bunkers continued. Several assaults were repelled during the night and in the early morning of 10 September,. At about 11.00, German engineers, supported by tanks and artillery, finally managed to destroy all but two of the Polish bunkers. Both of these were located in the centre of Strękowa Góra and continued fighting despite the fact that most of their men were wounded or incapacitated, and most of their machine guns destroyed. After the war, the Poles insisted that Guderian, in an attempt to end Polish resistance, threatened the Polish commander that he would shoot prisoners if the remaining forces did not surrender. The resistance continued for another hour, however, until a German officer arrived under a flag of truce and proposed a ceasefire. This lasted until about 13.30. Realising that all of his men were wounded and his ammunition almost wholly expended, Raginis ordered his men to surrender. He himself, already seriously injured, refused to surrender and committed suicide with a grenade.

After the Polish resistance had ended, the XIX Corps (mot.) advanced toward Wysokie Mazowieckie and Zambrów, where it fought the 'Battle of Zambrów' against the Polish 18^th Division, finally encircling and destroying that formation during the 'Battle of Andrzejewo'. The corps later advanced farther to the south and took part in the 'Battle of Brześć'.

Although all the bunkers were destroyed and the Polish resistance was finally broken, the Wizna fortified area had managed to halt the German advance for three days.

The exact number of Polish losses is unknown, mostly because there had survived very little information about the men taken prisoner by the Germans. Out of 720 Polish soldiers, only some 70 survived. Some managed to withdraw successfully and reach the Polish lines, but most of the living were taken prisoner. Some of these were subsequently killed by the Germans, others were beaten and abused but survived and were eventually delivered to prisoner of war camps.

The German losses are also unknown in any detail, an official release of the time mentioning only 'several dozen dead'. The Germans lost at least 10 tanks and several other armoured vehicles.