In August, as tensions with Germany were again heightened, this time over territorial demands on Poland, the French and British navies discussed coordination in the event of war with Germany; they agreed that the French would be responsible for covering Allied shipping south from the English Channel to the Gulf of Guinea in central Africa.
To protect shipping from German commerce raiders, the French created the Force de Raid (Raiding Force), with Dunkerque and Strasbourg as its core.
The group, which was under Gensoul's command, also included three light cruisers and eight large destroyers, and was based in Brest.
British observers informed the French Navy that the German "pocket battleships" of the Deutschland class had gone to sea in late August, bound for the Atlantic, and that contact with the vessels had been lost.
On 2 September, the day after Germany invaded Poland, but before France and Britain declared war, the Force de Raid sortied from Brest to guard against a possible attack by the Deutschlands.
Upon learning that the German ships had been spotted in the North Sea, Gensoul ordered his ships to return to point after they met the French passenger liner Flandre in the Azores.
The Force de Raid escorted the liner back to port and arrived in Brest on 6 September.

By this time, the German raiders had broken out into the Atlantic, so the British and French fleets formed hunter groups to track them down; the Force de Raid was split, with Dunkerque and Strasbourg operating individually as Force L and Force X, respectively.
Dunkerque, Béarn, and three cruisers remained in Brest while Strasbourg and two French heavy cruisers joined the British aircraft carrier HMS Hermes, based in Dakar.
On 22 October, Dunkeque and two cruisers sortied with their destroyer screen to cover Convoy KJ 3 to Kingston, Jamaica, arriving back in Brest three days later.
On 25 November, they again went to sea, joining the battlecruiser HMS Hood for a patrol to try to catch the German battleships Scharnhorst and Gneisenau, which had just sunk the British armed merchant cruiser HMS Rawalpindi.
While cruising off Iceland, Dunkerque ran into very heavy seas; her bow was repeatedly submerged by the large waves, and she had to slow to 10 knots (19 km/h; 12 mph) to avoid damage.
The Germans had by this time returned to port, so the Anglo-French force returned to its ports.

On arriving in Brest on 3 December, Dunkerque was docked for repairs to her bow.
The experience off Iceland had highlighted defects with her design, including insufficient freeboard and insufficiently strong construction.
These problems could not be easily corrected, however, leaving the Dunkerques vulnerable to storm damage.
On 11 December, Dunkerque and the cruiser Gloire carried a shipment of part of the Banque de France's gold reserve to Canada.
The ships arrived on 17 December and covered a convoy of seven troop ships that was also escorted by the battleship HMS Revenge on the return voyage.
While still at sea on 29 December, the convoy met the local escort forces in the Western Approaches, which took over responsibility of the convoy, allowing Dunkerque and Gloire to detach for Brest.
After arriving there on 4 January 1940, Dunkerque underwent another period of maintenance that lasted until 6 February.
The ship then conducted sea trials and training maneuvers from 21 February to 23 March.

Faced with increasingly hostile posturing by Italy during the spring of 1940, the Force de Raid was dispatched to Mers-el-Kébir on 2 April.
Dunkerque, Strasbourg, two cruisers, and five destroyers left that afternoon and arrived three days later.
The squadron was quickly ordered to return to Brest just a few days later in response to the German landings in Norway on 9 April.
The Force de Raid left Mers-el-Kébir that day and arrived in Brest on the 12th, with the intention that the ships would cover convoys to reinforce Allied forces fighting in Norway.
But the threat of Italian intervention continued to weigh on the French command, which reversed the ships' orders and transferred them back to
In August, as tensions with Germany were again heightened, this time over territorial demands on Poland, the French and British navies discussed coordination in the event of war with Germany; they agreed that the French would be responsible for covering Allied shipping south from the English Channel to the Gulf of Guinea in central Africa. To protect shipping from German commerce raiders, the French created the Force de Raid (Raiding Force), with Dunkerque and Strasbourg as its core. The group, which was under Gensoul's command, also included three light cruisers and eight large destroyers, and was based in Brest. British observers informed the French Navy that the German "pocket battleships" of the Deutschland class had gone to sea in late August, bound for the Atlantic, and that contact with the vessels had been lost. On 2 September, the day after Germany invaded Poland, but before France and Britain declared war, the Force de Raid sortied from Brest to guard against a possible attack by the Deutschlands. Upon learning that the German ships had been spotted in the North Sea, Gensoul ordered his ships to return to point after they met the French passenger liner Flandre in the Azores. The Force de Raid escorted the liner back to port and arrived in Brest on 6 September.
By this time, the German raiders had broken out into the Atlantic, so the British and French fleets formed hunter groups to track them down; the Force de Raid was split, with Dunkerque and Strasbourg operating individually as Force L and Force X, respectively. Dunkerque, Béarn, and three cruisers remained in Brest while Strasbourg and two French heavy cruisers joined the British aircraft carrier HMS Hermes, based in Dakar. On 22 October, Dunkeque and two cruisers sortied with their destroyer screen to cover Convoy KJ 3 to Kingston, Jamaica, arriving back in Brest three days later. On 25 November, they again went to sea, joining the battlecruiser HMS Hood for a patrol to try to catch the German battleships Scharnhorst and Gneisenau, which had just sunk the British armed merchant cruiser HMS Rawalpindi. While cruising off Iceland, Dunkerque ran into very heavy seas; her bow was repeatedly submerged by the large waves, and she had to slow to 10 knots (19 km/h; 12 mph) to avoid damage. The Germans had by this time returned to port, so the Anglo-French force returned to its ports.
On arriving in Brest on 3 December, Dunkerque was docked for repairs to her bow. The experience off Iceland had highlighted defects with her design, including insufficient freeboard and insufficiently strong construction. These problems could not be easily corrected, however, leaving the Dunkerques vulnerable to storm damage. On 11 December, Dunkerque and the cruiser Gloire carried a shipment of part of the Banque de France's gold reserve to Canada. The ships arrived on 17 December and covered a convoy of seven troop ships that was also escorted by the battleship HMS Revenge on the return voyage. While still at sea on 29 December, the convoy met the local escort forces in the Western Approaches, which took over responsibility of the convoy, allowing Dunkerque and Gloire to detach for Brest. After arriving there on 4 January 1940, Dunkerque underwent another period of maintenance that lasted until 6 February. The ship then conducted sea trials and training maneuvers from 21 February to 23 March.
Faced with increasingly hostile posturing by Italy during the spring of 1940, the Force de Raid was dispatched to Mers-el-Kébir on 2 April. Dunkerque, Strasbourg, two cruisers, and five destroyers left that afternoon and arrived three days later. The squadron was quickly ordered to return to Brest just a few days later in response to the German landings in Norway on 9 April. The Force de Raid left Mers-el-Kébir that day and arrived in Brest on the 12th, with the intention that the ships would cover convoys to reinforce Allied forces fighting in Norway. But the threat of Italian intervention continued to weigh on the French command, which reversed the ships' orders and transferred them back to 
French battleship Dunkerque