A small forested island separated from Gdansk by the harbour channel, Westerplatte was established as a Polish military outpost during the interwar period.

Despite initial assessment on both sides that the Polish garrison might hold out for several hours before being reinforced or overwhelmed, the Poles held out for seven days and repelled thirteen assaults that included dive-bomber attacks and naval shelling.

The defence of the Westerplatte was an inspiration for the Polish Army and people in the face of German advances elsewhere and is still regarded as a symbol of resistance in modern Poland.

The Free City was nominally supervised by the League of Nations but Danzig became increasingly allied with Germany, reflecting its predominantly ethnic German population.

[4]: 210 [5]: 21 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.

[9] The depot, referred to in League documents as the Depot for Polish Munitions in Transit in the Port of Danzig [pl] (WST)[10]: 45  (Polish: Wojskowa Składnica Tranzytowa), was completed in November 1925, officially transferred to Poland on the last day of that year, and became operational shortly after in January 1926, with 22 active storage warehouses.

The Polish garrison's complement was set at 2 officers, 20 NCOs, privates to a total strength of 88 all ranks, and Poland was prohibited construction of further military installations or fortifications on the site.

Most buildings were constructed with reinforced concrete and were supported by a network of field fortifications, including trenches, barricades and barbed wire.

[7][13][14]: 54 In March 1939, a German ultimatum to Lithuania led to Germany's annexation of the nearby Lithuanian coastal Klaipėda region; subsequently, the Westerplatte garrison was placed on alert.

[8]: 445  In addition to light arms consisting of pistols, grenades, and about 160 rifles, weaponry included a 75 mm field gun wz.

The outer line included entrenched outposts (codenamed Prom, Przystań, Łazienki and Wał) which were to hold long enough for the garrison to mobilize.

[19] On 25 August 1939, the German pre-dreadnought battleship Schleswig-Holstein, under the pretext of making a courtesy call, sailed into Danzig harbour,[8]: 446  anchoring 150 metres (160 yd) from Westerplatte.

Initially, the marines were ordered to attack on the morning of 26 August 1939, on that day Kleikamp moved the battleship farther upstream, and as a result, Sucharski put his garrison on heightened alert.

[19] The battle was fought at the semi - fortified supply depot on the Westerplatte peninsula at the mouth of the Vistula river north of Danzig (Gdansk).

[27]: 152  Shortly after, on Westerplatte, Sucharski radioed the nearby Polish military base on the Hel Peninsula, "SOS: I'm under fire.

[28]: 100 Eight minutes later Henningsen's marines from the Schleswig-Holstein, who had disembarked two hours earlier on the eastern side of the peninsula, advanced, expecting an easy victory over the Poles.

[19][28]: 100  However, after crossing the artillery-breached brick wall at the border, advancing about 200 metres (220 yd), and engaging the Polish Prom outpost, the Germans ran into an ambush.

[28]: 101  Around that 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.

[28]: 101  In that initial engagement, Poles sustained two casualties, and a Polish soldier, Staff Sergeant Wojciech Najsarek, was killed by machine-gun fire.

[35] Though the Poles never landed a hit on the German naval units, T196 and Schleswig-Holstein suffered accidents due to crew error or equipment failure, with at least one fatality and several injured men on the battleship.

[21] On 5 September, Sucharski held a conference with his officers, during which he urged surrender: the post had only been supposed to hold out for twelve hours.

At 03:00 on 6 September, during one of the attacks, the Germans sent a burning train toward the Polish positions, but the ploy failed when the terrified driver decoupled prematurely.

[18]: 14  The train failed to reach its target, an oil cistern; instead, it set fire to the woods, which had provided the Poles with valuable cover.

[36] At a second conference with his officers, on 6 September, Sucharski was again ready to surrender: the German Army was by now outside Warsaw, and Westerplatte was running critically low on supplies; moreover, many of the wounded were suffering from gangrene.

[36] Contemporary English-language publications which reported on the event, such as Life and the Pictorial History of the War, misidentified the Polish commander as a Major "Koscianski".

[37][19] On 8 September, the day after the capitulation, the Germans discovered a grave with the bodies of four unidentified Polish soldiers who had been executed by their comrades for attempted desertion.

[8]: 447  It tied up substantial German forces for much longer than anyone had expected, preventing Schleswig-Holstein from lending fire support in the nearby battles of Hel and Gdynia.

[54] Westerplatte is a common venue for state remembrance ceremonies relating to World War II, usually held on 1 September.

[63] Following the fall of communism in Eastern and Central Europe, a change symbolic of Poland's political transformation was the 2007 transfer of the Soviet T-34 tank from the cemetery to a museum in another town.