'Wasserkante' was the German bombing of Warsaw, the capital of Poland, as the decisive final element in the German siege of Warsaw in 'Weiss' (i) (25 September 1939).

The siege of Warsaw included great aerial bombardments of the city by the Luftwaffe starting on 1 September 1939. Land fighting started on 8 September, when the first German armoured units reached the Wola area and the south-western suburbs of the city. Here the German offensive was checked, and soon after this Warsaw was taken under siege. The siege lasted until 28 September, when the Polish garrison under Generał dywizji Walerian Czuma surrendered. The following day approximately 100,000 Polish soldiers left the city and entered captivity. On 1 October the German forces entered Warsaw.

From the start of 'Weiss' (i), Warsaw was the target of unrestricted German bombing. As well as military installations and industries, the bombing targeted civilian facilities such as water works, hospitals, market places and schools. In addition, civilians were strafed from the air with machine gun fire in what became known as a terror bombing campaign.

The anti-aircraft defences of the city were divided into active and passive parts. The former comprised mostly the units of the Fighter Brigade commanded by Pułkownik Stefan Pawlikowski, and the anti-aircraft gun and machine gun units commanded by Pułkownik Kazimierz Baran: the Fighter Brigade mustered 54 fighters, most of them of the PZL P.7 and P.11 types, while the anti-aircraft artillery had 86 guns as well as an unknown number of machine guns; and the latter was composed mostly of firefighting brigades and volunteers, and was supervised by Pułkownik Tadeusz Bogdanowicz and Julian Kulski, the deputy president of Warsaw.

Initially the air defence of Warsaw was fairly successful. By 6 September 1939 the Fighter Brigade had managed to shoot down 43 German aircraft, while the anti-aircraft artillery had shot down a similar number of bombers; there were also nine unconfirmed victories and claims for damage to another 20 aircraft. However, the Fighter Brigade had also suffered losses, and by 7 September was down to 16 aircraft, or some 30% of its initial strength. The anti-aircraft defences started to crumble when, on 5 September 5, 11 batteries were withdrawn toward Lublin, Brześć and Lwów.

As the campaign progressed, moreover, the German high command added significantly to the bomber effort directed at Warsaw. At the peak of the initial bombing campaign on 10 September, there were 17 consecutive attacks.

On 3 September Generalleutnant Georg-Hans Reinhardt’s 4^th Panzerdivision had broken through the positions of Generał dywizji Juliusz Karel Wilhelm Józef Rómmel’s Łódź Army near Częstochowa and began to drive forward to the Vistula river and Warsaw. On the same day Marszalek Polski Edward Śmigły-Rydz, the Polish commander-in-chief, ordered the creation of an improvised Command of the Defence of Warsaw (Downstage Obrony Warszawy) under the command of Czuma, head of the Border Guard (Straż Graniczna), with Pułkownik Tadeusz Tomaszewski as its chief-of-staff. Initially the forces under Czuma’s command were small, for in addition to most of the city authorities and a large part of the police force and fire-fighting service, much of the city’s military garrison had been withdrawn, leaving Warsaw with only four battalions of infantry and one battery of artillery. Moreover, the spokesman of the Warsaw garrison issued a communiqué in which all young men were to leave Warsaw.

To co-ordinate civilian efforts and counter the panic that started in Warsaw, Czuma appointed Stefan Starzyński, the mayor of Warsaw, as the Civilian Commissar of Warsaw. Starzyński started to organise a Civil Guard to replace the evacuated police forces and fire-fighters. He also ordered all members of the city’s administration to resume their jobs. In his daily radio releases he asked all civilians to construct barricades and anti-tank barriers at the outskirts of Warsaw. On 7 September Podpułkownik Józef Kalandyk’s 40^th Regiment 'Children of Lwów', which was passing through Warsaw to take up position with the Army Pomorze, was commandeered to join the defence of Warsaw. Polish field fortifications were constructed mostly west of the city, and Czuma’s forces were gradually reinforced by volunteers, as well as rearguard troops and units withdrawing from the front.

On the morning of 8 September the suburbs of Grójec, Radziejowice, Nadarzyn, Raszyn and Piaseczno were captured by forces of General Erich Hoepner’s XVI Corps (mot.), and at 17.00 the 4^th Panzerdivision attempted an assault on Warsaw’s western borough of Ochota. The assault was repulsed with heavy German losses. On the next day the division was reinforced with artillery and motorised infantry, and started another assault toward Ochota and Wola. The combination of well-placed Polish anti-tank guns and barricades on the main streets also managed to repel this assault. On many occasions the Polish troops' lack of armament had to be made up by ingenuity. One of the streets leading toward the city centre was covered with turpentine from a nearby factory. When the German tanks approached, the turpentine was set alight and the tanks were destroyed without a single shot. The German forces suffered heavy casualties and had to retreat. The 4^th Panzerdivision alone lost some 80 of the 220 of so armoured vehicles which began the assault.

By this time Czuma had gathered the equivalent of two infantry divisions under his command. His forces were supported by 64 pieces of artillery and 33 light tanks and tankettes.

On 8 September Śmigły-Rydz ordered the creation of an improvised Armia Warsaw (Warsaw Army) under Rómmel’s command. The newly created formation comprised the forces defending Warsaw and the Modlin Fortress, as well as all Polish units defending the lines of the Narew and Vistula rivers between the Warsaw and Pilica river lines. Czuma continued as commander of the Warsaw Defence Force, which he split into two sectors: East (Praga) under Podpułkownik Julian Janowski and West under Pułkownik Porwit. The Army Poznań under Generał diwizji Tadeusz Kutrzeba and the Army Pomorze under Generał brygady Władysław Bortnowski moved over to the offensive on the left flank of the German forces advancing toward Warsaw. As a result of this offensive, later known as the Battle of Bzura, the German command redeployed the 4^th Panzerdivision from Warsaw against the Polish threat near Kutno. The 4^th Panzerdivision was replaced by Generalleutnant Rudolf Kämpfe’s less capable 31^st Division.

In this sense the desperate attempt to buy time for organisation of defence of Warsaw was a success. The defenders of the city were joined by various units of the routed Army Prusy. In addition, several new units were created in Warsaw from the reserves of the Warsaw-based 8^th Division and 36^th 'Academic Legion' Regiment. On 11 September the Polish commander-in-chief ordered that Warsaw was to be defended at all costs. The following day the forces of General Georg von Küchler’s 3^rd Army broke through Polish lines along the Narew river and started its march south to isolate Warsaw from the east. The German formation was attacked by cavalry units under Generał brygady Władysław Anders, but after heavy fighting the Polish counter-offensive failed and their forces withdrew south. Other Polish units fighting under Generał brygady Juliusz Zulauf in the Narew river area retreated and reached Warsaw on 14 September, and were incorporated as the core of the defence forces of the borough of Praga.

On 15 September the German forces reached Warsaw from the east and the capital of Poland was under siege. Only a strip of land along the Vistula leading toward the Kampinos forest and the fortress of Modlin was still in Polish hands. On 16 September General Johannes Blaskowitz’s 8^th Army tried to capture Praga on the march, but was repulsed. After heavy fighting for the Grochów area the 23^rd Regiment was annihilated by the 21^st 'Children of Warsaw' Regiment under Pułkownik Stanisław Sosabowski. After the Battle of Bzura had ended, the remnants of the Poznań Army and the Pomorze Army broke through German encirclement and arrived in Warsaw and Modlin. After that the forces of the defenders amounted to approximately 120,000 soldiers.

The German forces preparing for an all-out assault numbered some 175,000 men. On 22 September the last lines of communication between Warsaw and Modlin were cut when the German forces reached the Vistula. Warsaw was now shelled by day and night, the artillery bombardment being supplemented by bombing, itself using the resources of General Albert Kesselring’s Luftflotte I and General Alexander Löhr’s Luftflotte IV.

From 20 September the German forces on the eastern bank of the Vistula river started daily attacks on Praga, but all were held off by the Polish forces. On 24 September all of the German formations concentrated around Warsaw were put under the command of Blaskowitz. On 25 September the final German preparations began, and in the early hours of the following day the Germans started a general assault on all sides of surrounded Warsaw. The western parts of the city were attacked by five German divisions (Generalleutnant Conrad von Cochenhausen’s 10^th Division, Generalleutnant Friedrich-Karl Cranz’s 18^th Division, Generalleutnant Günther Schwantes’s 19^th Division, the 31^st Division and Generalleutnant Paul von Hase’s 46^th Division) while the eastern part was attacked by four divisions (Generalleutnant Max Bock’s 11^th Division, Generalleutnant Franz Böhme’s 32^nd Division, Generalleutnant Siegfried Hänicke’s 61^st Division and Generalleutnant Richard Baltzer’s 217^th Division). The attacks were supported by some 70 batteries of field artillery, 80 batteries of heavy artillery and two entire air fleets, which bombarded the city continuously causing heavy losses among the civilian population.

The German air force flew about 1,150 bombing sorties against Warsaw on 25 September in 'Wasserkante' in an effort to terrorise the defenders into surrendering: 500 tons of high explosive bombs and 72 tons of incendiary bombs were dropped on the city.

The land attack was repelled and the German forces had to fall back to their initial positions. On the following night the Polish forces managed a successful counterattack which destroyed several German outposts near the Polish positions in boroughs of Mokotów and Praga. On 27 September the German command organised yet another all-out assault that was yet again repelled with heavy casualties on both sides. The military situation of Warsaw was relatively good. Czuma had managed by this juncture to gather enough forces and war matériel to defend the city for several weeks longer. However, the situation of the civilian inhabitants of Warsaw was becoming daily more tragic as constant bombardment of civilian facilities, lack of food and medical supplies resulted in heavy casualties among the civilians. The city’s water works were destroyed by German bombers and all boroughs of Warsaw experienced a lack of both potable water and water with which to extinguish the fires caused by the bombardment.

Moreover, the strategic situation for Poland was becoming very difficult, for the USSR had invaded Poland from the east and there was no support from the Western Allies, and this made further defence of the city pointless. On 26 September Kutrzeba, deputy commander of Warsaw, started surrender talks with the German commander, and at 12.00 on 27 September a ceasefire agreement was signed and all fighting halted. Soon after this Warsaw capitulated. Several units were unwilling to lay down their weapons, and their commanding officers had to be visited personally by Czuma or Rómmel. On 29 September the garrison of Warsaw started to hide or destroy their heavier weapons, and some of the hidden matériel was later used during the Warsaw uprising of 1944.

On 30 September the movement of Polish prisoners of war to German camps started, and on the following day German units entered the city. The siege of Warsaw was over.