The 'Battle of the Bzura River', otherwise known as the 'Battle of Kutno', was the largest Polish counter-offensive against the German 'Weiss' (i) invasion of Poland (9/19 September 1939).

The battle took place in the area to the west of Warsaw, near the Bzura river. It began as a Polish counter-offensive, which gained a measure of initial success, but the Germans then outflanked the Polish forces with a concentrated counterattack. This weakened the Polish forces and the Armia 'Poznań' and Armia 'Pomorze' were destroyed, leaving western Poland under German control. The battle has been described as 'the bloodiest and most bitter battle of the entire Polish campaign'.

The Polish plan for defence against a German invasion was 'Plan West', which called for a major defensive effort close against the Polish border with Germany. This was dictated more by political objectives than military reality as the Polish government feared that the Germans, after taking the territories which they lost in the Treaty of Versailles in 1919 after the end of World War I, would try to end the war and keep those territories. While defending the borders was riskier, the Poles were counting on timely British and French support which did not materialise, and indeed could not have been delivered. As a result of this, Generał dywizji Władysław Bortnowski’s Armia 'Pomorze' found itself in the 'Polish corridor' to the south of Danzig, surrounded by German forces on two fronts, and Generał dywizji Tadeusz Kutrzeba’s Armia 'Poznań' was pushed to the western edge of the 2^nd Polish Republic, separated both from its primary defensive positions and from other Polish armies.

The German offensive proved the folly of the border defence plan within the first days of the war. The Armia 'Pomorze' was defeated in the 'Battle of Bory Tucholskie' and forced to retreat to the south-east. The Armia 'Poznań', meanwhile, although not facing heavy German assault, was forced to retreat to the east as a result of the defeats of its neighbours, namely the Armia 'Pomorze' to its north and the Armia 'Łódź' to its south. The latter were retreating, meaning that the Armia 'Poznań' was threatened with being flanked and surrounded by the German forces. On 4 September, the Armia 'Poznań' moved through Poznań and abandoned this major city to the Germans despite the fact that the army was at this point not in contact with any significant German forces. By 6 September, the Armia 'Pomorze' and Armia 'Poznań' had linked, forming the strongest Polish operational formation of the campaign, and Bortnowski accepted the command of Kutrzeba.

On 7 September, the Polish forces became aware of the German push toward Łęczyca. If successful, this could cut off the line of retreat for the Polish forces. By 8 September, the German advanced formations had reached Warsaw and started to lay siege to Poland’s capital. At the same time, German forces had lost contact with the Armia 'Poznań', and the German command assumed that the army must have been moved by rail to aid in the defence of Warsaw, and thus remained unaware of the fact that the Armia 'Poznań' had merged with the Armia 'Pomorze', which the Germans considered, since its defeat at Bory Tucholskie, no longer to be a significant threat. On 8 September the Germans were certain that they had eliminated major Polish resistance in the area to the west of the Vistula river and were preparing to cross this and engage the Polish forces on the river’s eastern side.

Meanwhile, Kutrzeba and his staff had suspected, even before the German invasion, that it would be the neighbouring armies which would bear the brunt of the German invasion, and had therefore developed plans at an offensive toward the south in order to relieve the Armia 'Łódź'. In the first week of the campaign, those plans were rejected by the Polish commander-in-chief, Marszałek Polski Edward Rydz-Śmigły. By 8 September, Kutrzeba had lost contact with Rydz-Śmigły, who had relocated his command centre from Warsaw to Brest-Litovsk. As a result of these factors, Kutrzeba decided to proceed with his plan. His situation was dire, however, as the German forces were close to encircling his formations: General Johannes Blaskowitz’s 8^th Army of Generaloberst Gerd von Rundstedt’s Heeresgruppe 'Süd' had secured the southern bank of the Bzura river and General Günther von Kluge’s 4^th Army of Generaloberst Fedor von Bock’s Heeresgruppe 'Nord' had secured the northern bank of the Vistula river between Włocławek and Wyszogród, and some of its elements were attacking the rear of the Armia 'Pomorze' and Armia 'Poznań' from the direction of Inowrocław and crossing the Vistula river near Płock.

In the forthcoming battle, the Polish forces comprised the Armia 'Poznań' and Armia 'Pomorze' and the German forces Blaskowitz’s 8^th Army and General Walther von Reichenau’s 10^th Army of Heeresgruppe 'Süd', elements of von Kluge’s 4^th Army of Heeresgruppe 'Nord' of Heeresgruppe 'Nord'. Major air support was provided by formations of General Albert Kesselring’s Luftflotte I and General Alexander Löhr’s Luftflotte IV.

The 'Battle of the Bzura River' developed in three stages: in the first, between 9 and 12 September, the Poles attacked toward Stryków, aiming at the flank of the 10^th Army; in the second, between 13 and 13 September, the Poles attacked toward Łowicz; and in the third, between 16 and 19 September, the Germans counterattacked and eventually defeated the Poles, who withdrew toward Warsaw and Modlin.

On the night of 9/10 September, the Armia 'Poznań' began its counterattack from the area to the south of the Bzura river, its target being the German forces of the 8^th Army which were advancing between Łęczyca and Łowicz toward Stryków. Kutrzeba had noted that the 8^th Army was weakly secured in the north by only Generalleutnant Kurt von Briesen’s 30^th Division, which was stretched over an 18.5-mile (30-km) defensive line while the rest of the army was advancing toward Warsaw. The main thrust of the Polish offensive was delivered by forces under the command of General brygady Edmund Knoll-Kownack and known as the Operational Group 'Knoll-Kownacki' and comprising the 14^th, 17^th, 25^th and 26^th Divisions. The right wing of the offensive, in the Łęczyce area, included Pułkownik L. Strzelecki’s 'Podolska' Cavalry Brigade, and on the left, advancing from Łowicz toward the area of Głowno, Generał brygady Roman Abraham’s 'Wielkopolska' Cavalry Brigade. These groups inflicted considerable losses on the 30^th Division and Generalleutnant Friedrich Olbricht’s 24^th Division: sone 1,500 Germans were killed or wounded, and about 3,000 more were taken prisoner in this initial Polish push. The cavalry brigades, their strength supplemented by TKS and TK-3 light reconnaissance tanks, moved to threaten the flanks and rear of the advancing Germans.

The German forces were driven back some 12.5 miles (20 km) and the Poles recaptured several towns, including Łęczyca and Piątek, and also the village of Góra Świętej Małgorzaty. On 10 September, the Polish 17^th Division met Generalleutnant Herbert Loch’s German 17^th Division at Małachowicze, and on the following day the Polish forces continued their attack, advancing on Modlna, Pludwiny, Osse and Głowno.

Having initially underestimated the Polish advance, the Germans decided on 11 September to redirect the main force of the 10^th Army, the 4^th Army, the reserves of Heeresgruppe 'Süd' and aircraft of Luftflotte IV toward the Bzura river. These forces included Generalleutnant Rudolf Schmidt’s 1^st Panzerdivision, Generalleutnant Georg-Hans Reinhardt’s 4^th Panzerdivision and the newly formed Leibstandarte SS Adolf Hitler (mot.). German air superiority had a significant impact, making it very costly and difficult for the Poles to move formations and units in daylight. On the following day the Poles reached the line between Stryków and Ozorków, and on this same day Kutrzeba learned that units of the Armia 'Łódź' had retreated to the fortress of Modlin and decided to stop the offensive, instead planning to break through Sochaczew and the Kampinos forest to reach Warsaw.

During the morning of 14 September, Bortnowski’s 26^th and 16^th Divisions crossed the Bzura river in the area of Łowicz, and the 4^th Division reached the road linking Łowicz and Głowno. At this point however, Bortnowski ordered the 26^th Division to retreat, for he had learned of the withdrawal of the 4^th Panzerdivision from the outskirts of Warsaw and was concerned that this Panzer division posed a threat to his forces.

On 15 and 16 September, the Armia 'Pomorze' took up defensive positions on the northern bank of the Bzura river. General brygady Stanisław Grzmot-Skotnicki’s group was located between Kutno and Żychlin, General brygady’s Michał Karaszewicz-Tokarzewski’s units were near Gąbin, and parts of the Armia 'Poznań' were by the Bzura river near Sochaczew, and ready to begin their drive toward Warsaw. To encircle and destroy the Polish forces, the Germans used most of the 10^th Army, including two Panzer, one motorised and three light divisions, equipped with a total of some 800 tanks. The attack from all sides on the Polish positions started on 16 September, with the support of the Luftwaffe. On 15 September the Poles were forced out of Sochaczew, a town on the Bzura river, and trapped in a triangle of Bzura, Vistula and the German forces. After crossing the Bzura river between Sochaczew and Brochów and engaging the 25^th Division, the 1^st Panzerdivision took Ruszki, but its advance was then halted. The Poles began to cross the Bzura river near its confluence with the Vistula river, to the north of Sochaczew, and retreated toward Warsaw. The Polish forces were forced to abandon most of their heavy equipment as they crossed the river, however. On 17 September, German heavy artillery was shelling the crossing point to the north of Brochów, and the largest air operation of the campaign began, with the Luftwaffe attacking the retreating Polish forces.

During the night of 17 September, the main forces of the Armia 'Poznań' attacked the German forces in an effort to break out of the German encirclement between Witkowice and Sochaczew. The 15^th Division and the 'Podolska' Cavalry Brigade again crossed the Bzura river at Witkowice. In Brochow, the 25^th and 17^th Divisions also crossed the Bzura river. The 14^th Division was concentrated in Łaziska. At the same time, the Armia 'Pomorze' moved toward the villages of Osmolin, Kierozia and Osiek.

In the morning the Germans launched their drive to the south along both banks of the Bzura river, supported by more than 300 aircraft and heavy artillery. The German howitzers, taking advantage of their position on the high ground of the Vistula river’s right bank, shelled Polish positions for the entire day. And after two days of heavy fighting, with no ammunition or food rations remaining, further Polish attempts at a break-out became impossible.

Only a few Polish units managed to break out of the German encirclement, and these entered Warsaw and Modlin, mostly around 19 and 20 September, crossing the Kampinos forest and fighting German units in the area in several engagements including, for example, the 'Battle of Wólka Węglowa'. Among those who broke out were Kutrzeba, Knoll-Kowacki and Tokarzewski, the 'Wielkopolska' and 'Podolska' Cavalry Brigades under Abraham, and the 15^th and 25^th Divisions. The rest of the Polish formations were the 4^th, 14^th, 17^th, 26^th and 27^th Divisions, and these did not manage to cross the river and, under Bortnowski’s command, surrendered between 18 and 22 September. The Polish casualties were estimated at 20,000 dead, including three generals: Franciszek Wład, Stanisław Grzmot-Skotnicki and Mikołaj Bołtuć. The German casualties are estimated at 8,000 dead.

After the 'Battle of the Bzura River', the remaining German divisions advanced as speedily as possible toward Warsaw and Modlin, and soon encircled both of these cities. The Bzura river campaign ended in defeat for the Poles, but because of the initial Polish local successes the German advance on Warsaw was delayed for several days. The Germans had to divert formations and units from their push toward Warsaw. This enabled the Polish units defending Warsaw and its area round it to organise their own longer-term, but ultimately impossible, defence of their capital.

The campaign also showed the importance of seizing the initiative, proved that horsed cavalry units were still an important factor on the battlefield, confirmed the importance of air superiority and confirmed that simple numerical superiority was not decisive.