The 'Battle of Westerplatte' was fought between German and Polish forces as the first battle of the German 'Weiss' (i) invasion of Poland (1/7 September 1939).

The battle took place on the Westerplatte peninsula in the harbour of the Free City of Danzig.

In the mid-1920s, the 2^nd Polish Republic established the Polish Military Transit Depot on the Westerplatte peninsula in the Free City of Danzig. Beginning on 1 September 1939, the German forces and the Danzig police attacked this and, despite each side’s initial assessment that the Polish garrison might hold out for several hours before being reinforced or overwhelmed, the Poles held out for seven days and repelled 13 assaults that included dive-bomber attacks and naval shelling. The defence of the Westerplatte was an inspiration for the Polish armed forces and the Polish people in the face of German advances elsewhere.

The Westerplatte is a peninsula in the Bay of Danzig (now Bay of Gdańsk). After the re-establishment of Polish independence in the aftermath of World War I, much of the surrounding region became part of Poland, while the port city of Danzig (now Gdańsk) became an independent city-state, the Free City of Danzig. This as nominally supervised by the League of Nations, but became increasingly allied with Germany, reflecting its predominantly ethnic German population.

In 1921, in the wake of the Polish-Soviet War, the League of Nations granted Poland the right to install a garrisoned ammunition depot near Danzig. Despite the Free City’s objections, this right was confirmed in 1925, and an area of 0.23 sq miles (0.6 km²) was selected on the Westerplatte peninsula. Westerplatte was separated from the port of Danzig mainly by the harbour channel, and on land the Polish-held part of Westerplatte was separated from Danzig’s territory by a brick wall topped with barbed wire. A dedicated railway line, passing through the Free City, connected the depot with nearby Polish territory. The depot, known in League documents as the Depot for Polish Munitions in Transit in the Port of Danzig, was completed in November 1925, was officially transferred to Poland on the last day of that year, and became operational in January 1926 with 22 storage warehouses. The Polish garrison’s complement was set at two officers, 20 non-commissioned officers and 66 enlisted men, and Poland was prohibited from the construction of further military installations or fortifications on the site.

By a time early in 1933, there were increasing German complaints about the need for border adjustments. In addition, the Polish and French governments discussed the need for a preventive war against Germany. On 6 March, in what became known as the 'Westerplatte incident' or 'Westerplatte crisis', the Polish government landed a marine battalion on Westerplatte, briefly reinforcing the local garrison to about 200 men and thereby demonstrating Polish resolve to defend the depot; the Polish undertaking was also intended to put pressure on the Danzig government, which was attempting to repudiate an earlier agreement on shared Danzig/Polish control over the harbour police and to acquire full control of the police and the harbour. One source avers that on 14 March 1933 the League of Nations had authorised Poland to reinforce its garrison, but another claims that the additional Polish troops were withdrawn on 16 March after protests from the League of Nations, Danzig and Germany, though only in exchange for Danzig’s withdrawal of its objections to the harbour and police agreement.

The Poles later built, on a clandestine basis, fortifications on Westerplatte. These were relatively minor: there were neither bunkers nor tunnels, but only several small guardhouses, partially hidden in the peninsula’s forest, and in the centre of the peninsula several more buildings including barracks. Most of these buildings were constructed of reinforced concrete and were supported by a network of field fortifications, including trenches, barricades and barbed wire entanglements.

In March 1939, a German ultimatum to Lithuania led to Germany’s annexation of the nearby Lithuanian coastal region of Memel (now Klaipėda), and the Westerplatte garrison was later placed on alert. Fearing a possible German coup d'état in Danzig, the Poles reached a secret decision to reinforce their Westerplatte garrison: the subterfuge employed was the departure from Westerplatte of Polish civilians in army uniform and their replacement by Polish troops. Thus by a time late in August 1939 the Poles had reinforced their 88-man garrison, though its increased strength is still debated: recent research suggests something in the order of 210 to 240 men including six officers: Major Henryk Sucharski, his second-in-command Kapitan Franciszek Dąbrowski, Kapitan Mieczysław Słaby, Porucznik Leon Pająk, Porucznik Stefan Ludwik Grodecki and Podporucznik Zdzisław Kręgielski. Estimates include some 20 mobilised civilians and about 10 regular troops who happened to be present when fighting began. In addition to personal arms, weapons included one 75-mm (2.95-in) field gun, two Bofors 37-mm anti-tank guns, four 81-mm (3.19-in) mortars and about 40 machine guns, the last including 18 heavy weapons. The field fortifications had also been extended with more trenches, wooden barricades, barbed wire strung into wire obstacles, and reinforced concrete shelters built into the basements of the barracks. Foliage was also thinned to reduce cover on the more likely axes of attack.

The Polish defence, which anticipated principally a German land-based assault, rested on three lines. The outer line included entrenched outposts codenamed 'Prom', 'Przystań', 'Łazienki' and 'Wał' designed to hold for a time long enough for the garrison to mobilise. The second line was based on five guardhouses (numbered I to V) in the centre of the depot. The final line comprised the headquarters and barracks at the depot’s centre and sometimes mentioned as Guardhouse VI. In addition, the Poles also had several supporting positions ('Elektrownia', 'Deika', 'Fort', 'Tor kolejowy' and 'Kej'). The defence plan called for the garrison to hold for 12 hours, after which the siege was expected to be lifted by reinforcements arriving from the mainland.

On 25 August, the German pre-dreadnought battleship Schleswig-Holstein, sailed into Danzig harbour on the pretext of a courtesy visit and anchored 165 yards (150 m) off Westerplatte. On board was a Marinestosstruppkompanie (marine shock-troop company) of 225 marines under Leutnant Wilhelm Henningsen. On land the Germans had the SS Heimwehr Danzig force of 1,500 men under a police officer, Generalleutnant Friedrich-Georg Eberhardt. In overall command was Kapitän zur See Gustav Kleikamp aboard Schleswig-Holstein.

Initially, the marines were ordered to attack on the morning of 26 August. On that day Kleikamp moved the battleship farther upstream, and as a result Sucharski put his garrison on heightened alert. Shortly before the German disembarkation, the orders were rescinded as Adolf Hitler had postponed the start of 'Weiss' (i) on learning of the Polish-British Common Defence Pact, signed the previous day, and that Italy was hesitant about its obligations under the 'Pact of Steel'.

Eberhardt and Kleikamp lacked specific information on the Polish defences, but assumed that the preliminary bombardment would soften the fortifications enough for the marines to capture Westerplatte. Kleikamp had been assured by the Danzig police that 'Westerplatte will be taken in 10 minutes'.  Eberhardt himself was more cautious, estimating that 'a few hours' would be needed to overcome the Polish garrison, which the Germans estimated at no more than 100 men.

On the early morning of 1 September, Schleswig-Holstein opened fire with her 283-mm (11.14-in) main guns on the Polish garrison at a time stated variously as 04.45, 04.47 or 04.48. Soon after this, on Westerplatte, Sucharski radioed the nearby Polish military base on the Hel peninsula 'SOS: I am under fire.' The battleship’s initial bombardment was not very successful and failed to inflict a single casualty among the defenders as the ship’s proximity to its target meant that the shells of its heavier guns lacked the time to arm themselves and therefore failed to detonate on impact.

Eight minutes later Henningsen’s marines from Schleswig-Holstein, who had disembarked two hours earlier on the western side of the peninsula’s isthmus, advanced in the expectation of an easy victory. However, after crossing the artillery-breached brick wall at the border, advancing about 220 yards (200 m) and engaging the Polish 'Prom' outpost, the Germans ran into an ambush. They now found themselves in a 'kill zone' of Polish crossfire from concealed firing positions, while barbed-wire entanglements impeded their movement. At about 05.15, the field gun, under the control of Pająk, opened an intense fire on the advancing Germans, firing 28 rounds and knocking out several machine gun nests above warehouses across the harbour canal.  Meanwhile, the German marines were also bombed by the Polish mortars, and even the battleship itself was targeted by the Polish 37-mm guns. At about this time, the Poles also repulsed an attempt by a small maritime unit of the Danzig police to land on the western side of the depot. In that initial engagement, Poles sustained two casualties, and one Polish soldier was killed by machine gun fire.

At 06.22, the German marines radioed the battleship that they had sustained heavy losses and were withdrawing: by this time the casualties were about 50 Germans and eight Poles, most of them wounded. A longer bombardment from the battleship between 07.40 and 08.55 preceded a second attack and succeeded at knocking out the Polish field gun.The German marines attacked again from 08.35 to 12.30 but encountered mines, felled trees, barbed wire and intense fire. By 12.00, when the Germans retreated, Henningsen had been gravely wounded. Eberhardt called for air support, but this was delayed by adverse weather over Westerplatte. On that first day’s combat, the Polish side had lost four men killed and several wounded, while the Germans had suffered 16 men killed and about 120 wounded.

The German commanders now arrived at the conclusion that a ground attack was not feasible until the Polish defences had been further softened. Re-examining aerial photographs, from which they had previously underestimated the Polish defences, they now overestimated them, concluding that the Poles had constructed extensive underground and armoured fortifications: for example, six haystacks were declared to be armoured bunker domes. In the following days, the Germans bombarded the Westerplatte peninsula with naval guns and heavy field artillery, including a 105-mm (4.13-in) howitzer battery and 210-mm (8.27-in) howitzers. Between 18.05 and 18.25 on 2 September, a two-wave air raid by 60 Junkers Ju 87 single-engined dive-bombers dropped 26.5 tonnes of bombs, eliminating the Polish mortars, destroying Guardhouse V with a 1,102-lb (500-kg) bomb and killing at least eight Polish soldiers. The air raid shrouded all of Westerplatte in smoke and destroyed the Poles' only radio and much of their food supply.  According to some German sources, after the air raid the Poles briefly displayed a white flag, but this is disputed.

On 4 September, the German torpedo boat T-196, supported by the elderly minesweeper von der Gronen, made a surprise attack from the seaward side of the peninsula, and the Poles abandoned their 'Wał' outpost. Now only the 'Fort' outpost prevented an attack from the north. Although the Poles never landed a hit on the German ships, T-196 and Schleswig-Holstein suffered accidents as a result of crew errors and equipment failure, with at least one fatality and several injured men on the battleship.

On 5 September, Sucharski held a conference with his officers, during which he urged surrender: he argued that the post had been intended to hold for only 12 hours. Sucharski’s deputy, Dąbrowski, opposed surrender and the group decided to hold for a while longer.

Subsequently, the Poles repelled several cautious German probing attacks. At 03.00 on 6 September, during one of these attacks, the Germans sent a burning train toward the Polish positions, but the ploy failed when the terrified driver decoupled prematurely and the train failed to reach its target, an oil tank, and instead set fire to the woods, which had provided the Poles with valuable cover. In addition, the burning wagons created a perfect field of fire for the Poles, and the Germans suffered heavy losses. A second fire train attack, in the afternoon, also failed.

At a second conference with his officers, on 6 September, Sucharski again argued for surrender: the German army was by now outside Warsaw, Westerplatte was running critically short of supplies, and many of the wounded were showing signs of gangrene. At 04.30 on 7 September, the Germans opened intense fire on Westerplatte, and thus lasted to 07.00. Flamethrowers and bombardment destroyed Guardhouse II and damaged Guardhouses I and IV.  Schleswig-Holstein also participated in the bombardments.

At 09.45 on 7 September, the Poles raised a white flag. The Polish defence had so impressed Eberhardt that initially he allowed Sucharski keep his ceremonial sabre, but this was later seized. Sucharski surrendered the post to Kleikamp, and the Germans stood at attention as the Polish garrison marched out at 11.30. More than 3,000 Germans, including soldiers and support formations such as the Danzig police, had been committed for one week against a small Polish garrison; about half of the Germans (570 on land and more than 900 at sea) had taken part in direct action. German casualties totalled 50 killed (16 of them naval) and 150 wounded. The Poles had lost 15 men killed and had sustained at least 40 wounded.

On 8 September, the day after the Polish capitulation, the Germans discovered a grave with the bodies of four unidentified Polish soldiers who had been executed by their comrades for attempted desertion. Five days after the capitulation, on 12 September 1939, the Polish wireless operator, Sergeant Kazimierz Rasiński, was murdered by the Germans after the had undergone a brutal interrogation during which he refused to hand over radio codes.  On 19 September Hitler came to visit Danzig and two days later inspected the Westerplatte.

The Westerplatte saw another round of fighting during the 'Vistula-Oder Strategic Offensive Operation' in 1945. Between 28 March and 1 April, elements of the 73^rd Division defended the peninsula against the 76^th Guards Division until the German units were evacuated by sea.

The 'Battle of Westerplatte' is often described as the first battle of World War II, but was in fact only one of several battles in the 'Battle of the Border' initial phase of the German invasion of Poland. For each side, the battle had been mostly of political, rather than tactical, significance. It had tied substantial German forces for much longer than anyone had expected, preventing Schleswig-Holstein from lending fire support in the nearby 'Battle of Hel' and 'Battle of Gdynia'.