| Conflict: | Battle of Gdynia |
| Partof: | Invasion of Poland |
| Date: | 8–14 September 1939 |
| Place: | Gdynia, Pomeranian Voivodeship, Poland |
| Result: | German victory |
| Commander1: | Fedor von Bock Leonhard Kaupisch |
| Commander2: | Józef Unrug Stanisław Dąbek |
| Strength1: | 29,000 |
| Strength2: | 17,000 |
| Casualties1: | Unknown |
| Casualties2: | Unknown |
Battle of Gdynia was one of the major battles in northern Poland during the Invasion of Poland of 1939. The Germans' main push towards Gdynia began on 8 September and they captured Gdynia six days later on 14 September.
Gdynia is a major civilian and military port on the Baltic Sea, and was an important industrial centre of the Second Polish Republic. Its defence was one of the key elements in the Polish defence plan. One of the Polish armies, Armia Pomorze, had orders to prevent German forces from breaking through the Polish Corridor that linked most of Germany with East Prussia and to defend Gdynia, Hel, Kępa Oksywska and Oksywie.
The forces defending Gdynia were grouped under the Land Coastal Defence (Lądowa Obrona Wybrzeża), commanded by Colonel Stanisław Dąbek and Naval Coastal Defence (Morska Obrona Wybrzeża) Captain Stanisław Frankowski. The Polish plan foresaw that the Germans would manage to cut off Gdynia from the main forces of Armia Pomorze; therefore, the Land Coastal Defence was tasked with defence of the coast for 8–10 days and promised reinforcements after that period. It was thus prepared for individual operations and reassigned from Armia Pomorze to the Polish Navy, and Stanisław Dąbek received orders directly from counter-admiral Józef Unrug. The Polish Navy would support the coastal defence with 1 destroyer ; 3 other destroyers had been evacuated from the Baltic Sea during Plan Peking), 1 heavy minelayer and several submarines and smaller surface ships.
On 1 September the Polish Land Coastal Defence force had a strength of approximately 17,000 (increased from about 5,000 two months earlier), with 40 artillery pieces (including 8 anti-aircraft), 34 mortars and grenade launchers and approximately 400 machine guns. Infantry units were placed:
The German forces tasked with the capture of Gdynia were commanded by General Leonhard Kaupisch and formed part of the Army Group North under General Fedor von Bock. They were approximately 29,000 strong, with over 300 artillery pieces, 70 mortars and grenade launchers and 700 machine guns. The Kriegsmarine (German navy) in the area consisted of 2 old battleships, 3 cruisers, 10 destroyers and several smaller units. Approximately 120 Luftwaffe (aircraft) planes were supporting the German forces.
The German offensive isolated the Polish coast from the Polish mainland between 4 and 8 September, and Armia Pomorze was forced to retreat south-east. Units of Land Coastal Defence has been engaged by the German forces from 1 September, although it was not until the second week of the war that the Germans would begin a direct assault on the Polish port of Gdynia.
The Kriegsmarine, operating in the area with the pre-dreadnought Schleswig-Holstein and smaller units shelled Polish positions, but with limited effect, and were prevented from closing on the coast by Polish coastal batteries.
The German land offensive was much more successful. German forces established a connection between Germany and Prussia by 4 September. After heavy fighting near Kartuzy (5 September), their main attack towards Gdynia began on 8 September, and after intense fighting near Puck and Wejherowo (8 September) step by step the Poles were forced back towards the sea.
When the Polish positions outside Gdynia were overrun, and in view of the retreat of the Armia Pomorze, on 10 September Colonel Dąbek decided to abandon Gdynia in order to avoid civilian casualties during the city fighting, and thus on 12 September he ordered all Polish units to retreat from Gdynia toward Kępa Oksywska, a fragment of the coast near Puck Bay. On 11 and 12 September the fights near Mechlinki transformed into an incessant two-day battle, after which Polish troops had to retreat again.
The Germans captured Gdynia on 14 September. Kępa Oksywska was not prepared for siege or provisioned for the approximately 9,000 strong remaining Polish forces and in the battle of Kępa Oksywska the Poles sustained heavy losses (approximately 2,000). After a last failed counterattack on 19 September, Colonel Dąbek, who participated in the front line battle and was wounded, decided to issue order to surrender and committed suicide after giving the order to cease fire.
The Polish Navy was tasked to provide support to Gdynia through Plan Worek (submarine operations) and Operation Rurka (laying a mine barrage in Gdańsk Bay). The five submarines assigned to Plan Worek left their war stations in early September. Three submarines faced internment in neutral ports. Two reached Great Britain. One mine set by a submarine sank a German minesweeper. As for Operation Rurka, a German air attack on September 1 left the minelaying vessel damaged, unused mines were dumped into the sea, and the vessel docked in the port in Hel where no mines were readily available, which led to cancellation of the operation.