The Thai fleet was centered on two Japanese-built armored coast defence vessels—HTMS Thonburi and Sri Ayudhya—which displaced 2,500 long tons (2,540 t) and carried four 8-inch (203 mm) guns in two twin-gun turrets.

In addition, the Royal Thai Air Force had on its rosters over 140 aircraft, including relatively modern Mitsubishi Ki-30 (Allied reporting name "Ann") light bombers, which saw extensive service against the French during the war.

Other less capable aircraft in the Thai inventory included P-36 Hawk fighters, 70 Chance-Vought O2U-2 Corsair biplanes, six Martin B-10 bombers, and several Avro 504 trainers.

Because of the disparate speeds of the French ships, Bérenger sent the slower sloops on ahead, while he remained in Saigon to complete the final elements of the plan.

The delay in starting the operation allowed him to refuel at Cape St. Jacques before his rendezvous with the slower ships at 16:00 on 15 January, 20 mi (17 nmi; 32 km) north of Poulo Condore.

On the evening of 15 January, following a last conference on board the flagship, the squadron weighed anchor at 21:15 and closed the Thai coast at 14 knots (16 mph; 26 km/h), the top speed of the sloops.

Once their presence had been passed to Lamotte-Picquet, the aircraft attempted a bombing attack, but were driven off by a heavy barrage of anti-aircraft fire.

Caught with their crews asleep, the Thai ships desperately began to raise steam and prepared to slip their anchors.

The cruiser also destroyed a shore observation post, preventing the Thai from quickly relaying information to their air forces at Chantaboun.

[5] At 06:38, lookouts aboard Lamotte-Picquet spotted the coastal defence ship HTMS Thonburi heading northwest, at a range of 10,000 metres (11,000 yd).

At the beginning of the engagement, a lucky shot from Lamotte-Picquet killed the captain of Thonburi, Commander Luang Phrom Viraphan, and disrupted her operations.

Believing they had a better chance of hurting the smaller French ships, the Thais shifted their fire onto Amiral Charner, which soon found 8-inch salvoes falling around her.

Thonburi shifted fire back to Lamotte-Picquet after a salvo from the French cruiser put her aft turret out of action.

She soon reached the safety of shallow water, which the French ships could not enter for fear of grounding, but Thonburi was already burning fiercely and listing heavily to starboard.

At 07:50, Lamotte-Picquet fired a final salvo of torpedoes at 15,000 metres (16,000 yd), but lost sight of Thonburi behind an island from which she was not seen to emerge.

On the other hand, the French sailors were elated, believing they had inflicted a decisive defeat while not suffering losses of significance, with only 11 men killed.

Fearing that any further action by the French might turn back the Thai invasion, the Japanese government offered to mediate a peaceful settlement.

However, even this interim of peace did not last long, as the Imperial Japanese Army invaded Thailand in December of that year as part of its attempt to capture British Malaya.