German battleship Schleswig-Holstein firing at point blank range during the Battle of Westerplatte

The Battle of Westerplatte was the first battle of the German invasion of Poland, marking the start of World War II in Europe. It occurred on the Westerplatte peninsula in the harbour of the Free City of Danzig (now Gdańsk, Poland).

Early on 1 September 1939, Germany launched an invasion of Poland. Schleswig-Holstein had been positioned in the port of Danzig, moored close to the Polish ammunition depot at Westerplatte under the guise of a ceremonial visit in August. Around 04:47 on 1 September, Schleswig-Holstein opened fire with her main battery at the Polish positions on the Westerplatte, and in doing so fired the first shots of World War II. These shots were the signal for ground troops to begin their assault on the installation,[ though the first German ground attack in the Battle of Westerplatte was repelled shortly thereafter.[ A second assault began later that morning, again supported by Schleswig-Holstein, though it too had failed to break into the installation by around noon.

Schleswig-Holstein was joined on 4 September by the torpedo boats T196 and Von der Gröben. A force of German infantry and army engineers went ashore to take the depot, with heavy fire support from Schleswig-Holstein. The Poles managed to hold off the Germans until they were forced to surrender on 7 September at 10:30. Following the Polish surrender, Schleswig-Holstein began shelling Polish positions at Hel and Redłowo; these operations lasted until 13 September. Between 25 and 27 September, the old battleship returned to Hel with her sister Schlesien; both vessels conducted further bombardments of Polish positions there. On 25 September the Schleswig-Holstein was lightly damaged by Polish coastal batteries at Hel.

The ship's primary armament consisted of four 28 cm SK L/40 guns in two twin turrets; one turret was placed forward and the other aft. Her offensive armament was rounded out with a secondary battery of fourteen 17 cm (6.7 in) SK L/40 guns mounted individually in casemates. A battery of twenty-two 8.8 cm (3.5 in) SK L/45 guns in single mounts provided defense against torpedo boats. The ship was also armed with six 45 cm (17.7 in) torpedo tubes, all below the waterline. One was in the bow, one in the stern, and four on the broadside. Her armored belt was 240 mm (9.4 in) thick amidships in the citadel, and she had a 40 mm (1.6 in) thick armored deck. The main battery turrets had 280 mm (11 in) thick sides.

0:19 shows both the muzzle blast and the impact of shells from the main battery, while the guns were outdated at the time they were still firing shells weighing around 250 kg at over twice the speed of sound and the effect on target was considerable.