The 'Battle of Ko Chang' was fought between Vichy French and Thai naval forces within the context of the 'Franco-Thai War' (October 1940/January 1941) and involved a flotilla of Vichy French warships in an attack on a smaller force of Thai vessels, including a coast-defence ship (17 January 1941).

The battle was a tactical success for the Vichy French navy over the Royal Thai navy, but its strategic result is disputed as the Japanese intervened diplomatically and mediated a ceasefire which was in Thailand’s favour as all disputed territories in French Indo-China were ceded to Thailand. Within a month of the engagement, the Vichy French and Thais negotiated a peace which ended the war.

As this time the 'Franco-Thai War' was proceeding quite poorly for the Vichy French land forces, which were ill-equipped to deal with the larger Royal Thai army. The French governor general of Indo-China and commander-in-chief of the local naval forces, Vice-amiral Jean Decoux, had earlier decided that Indo-China’s best chance at stopping Thai incursions lay in a naval attack on Thailand’s fleet and coastal cities, for this would clear the way for bombardment missions to support a counter-offensive along the Cambodian frontier.

Although comparatively small, the Royal Thai navy had been modernised with the recent acquisition of vessels from both Japan and Italy. The major units of the fleet included two Japanese-built armoured coast-defence vessels, each displacing some 2,265 tons and carrying 8-in (203.2-mm) guns, two older British-built armoured gunboats with 6-in (152.4-mm) guns, 12 torpedo boats and four submarines. In addition, the Royal Thai air force had more than 140 aircraft, including relatively modern Mitsubishi Ki-30 'Ann' single-engined light bombers, which saw extensive service against the Vichy French during the war. These aircraft were quite capable of disrupting any French naval operation which might be mounted. Other less capable aircraft in the Thai inventory included Curtiss P-36 Hawk single-engined fighters, 70 Vought O2U-2 Corsair single-engined biplane observation aircraft, six Martin B-10 twin-engined medium bombers, and several Avro Type 504 single-engined biplane trainers.

Decoux formed a small Groupe Occasionnel (ad hoc group) squadron on 9 December 1940 at Cam Ranh Bay, to the north of Saigon. In command was Capitaine de Vaisseau Régis Bérenger, and the squadron comprised the light cruiser Lamotte-Picquet built in 1923 and notable for its high speed but poor protection, the modern avisos (colonial sloops) Dumont d’Urville and Amiral Charner especially designed for colonial service, capable of embarking one infantry company, but only lightly armed and possessing no armour, and the older avisos Tahure and Marne. There was no air cover other than that which could be provided by nine Loire 130 single-engined reconnaissance flying boats based at Ream. Additional scouting capability was provided by three coastal survey craft and intelligence gleaned from local fishermen.

Bérenger’s squadron began training manoeuvres in Cam Ranh Bay shortly after assembling. On 13 January 1941, Decoux formally requested Bérenger to lead the squadron against the Royal Thai navy to support a land offensive planned for 16 January. The land action was meant to force back the Thai ground forces which had been advancing along the coast. Because of the disparate speeds of the French ships, Bérenger sent the slower sloops ahead, while he remained in Saigon to complete the final elements of the plan. Several options were being prepared, the admiralty in Vichy France having given its approval for the use of naval forces to support the army. The final planning meeting on 13 January saw an 24-hour immediate delay in the execution of the operation. Once the plans had been completed, Bérenger sailed in Lamotte-Picquet. 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 at a position 20 miles (32 km) to the north of Poulo Condore.

Decoux’s orders were simple: 'Attack the [Thai] coastal cities from Rayong to the Cambodian frontier to force [the Thai] government to withdraw its forces from the Cambodian frontier.' 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 kt, the sloops' top speed. The Vichy French ships remained undetected as they entered the Gulf of Siam, but their quarry was not as fortunate. The Loire 130 aircraft from Ream had completed a sweep of the coast from Trat to Sattahip, and sighted one coast-defence ship and one torpedo boat at Ko Chang, and one gunboat, four torpedo boats and two submarines at Sattahip.

The flying boats' report was forwarded to the naval headquarters in Saigon, which re-transmitted it to Lamotte-Picquet. Bérenger considered his options and decided on a dawn attack against the Thai ships at Ko Chang, ignoring Sattahip as the sloops would not be able to reach it until later in the day, when the element of surprise would already have been lost. Also, the strength of Sattahip’s harbour defences was unknown. The Thai naval force at Ko Chang was weaker and thus offered a better chance of victory.

Bérenger approached Ko Chang at dawn from the south-west. Because the anchorage was surrounded by islands and islets, many of them more than 660 ft (200 m) high, it had been decided that the squadron would divide and use the cover of the islands to concentrate fire on portions of the Thai squadron, while also covering all the avenues of escape. The easternmost channel was the most likely route by which a retirement would be made. It was the most suitable route and that in which the aerial reconnaissance had placed the largest Thai ships. Lamotte-Picquet would head to the eastern side of the anchorage to block the route, while the sloops blocked the centre and shelled the Thai ships there. The smaller French ships would concentrate to the west.

The French squadron closed the anchorage at 05.30 on 17 January. and 15 minutes later divided into the three planned groups: Lamotte-Picquet headed for the eastern part of the anchorage, Dumont d’Urville and Amiral Charner continued to the central position, and Tahure and Marne made for the western side. Conditions were perfect: the weather was fine, and the sea calm and almost flat. Sunrise was at 06.30, and the scene was lit only by the first rays of light on the horizon and by the dim moonlight.

A final aerial reconnaissance of the target area had been arranged, using one of the Ream-based Loire 130 flying boats. Lamotte-Picquet also carried two Gourdou-Leseurre GL-832 single-engined reconnaissance/spotter floatplanes, but these could not be launched as a result of catapult problems. At 06.05, the Loire 130 overflew the anchorage and reported two torpedo boats. This came as an unpleasant surprise for the French as earlier reports had led them to believe that only one was present, but during the night Chonburi had arrived to relieve Chantaburi, which was to return to Sattahip later that day for repairs.

Once the presence of the two Thai ships had been passed to Lamotte-Picquet, the aeroplane attempted a bombing attack, but was driven off by a heavy barrage of anti-aircraft fire. The effect of this mission was double edged. The French were now aware of what they faced, but the element of surprise had been lost and it was still 30 minutes to sunrise. Caught with their crews asleep, the Thai ships desperately began to raise steam and prepared to slip their anchors. However, both torpedo boats were soon sunk by heavy gunfire from Lamotte-Picquet's 155-mm (6.1-in) guns. The cruiser also destroyed a shore observation post, preventing the Thais from quickly relaying information to their air forces at Chantaboun.

At 06.38, look-outs on Lamotte-Picquet spotted the coast-defence ship Thonburi heading to the north-west at a range of 11,000 yards (10000 m). A running battle began, with the fire of both ships frequently blocked by the towering islets. The fire from the Thai ship was heavy but inaccurate. By 07.15, fires could be seen on Thonburi, which then found herself engaging not only by the cruiser but also the sloops. At the beginning of the engagement, a round from Lamotte-Picquet killed Thonburi's captain, 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-in (203-mm) salvoes falling round her.

Thonburi then shifted her fire back to Lamotte-Picquet after a salvo from the French cruiser put her after 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. Her remaining turret was jammed and could not fire unless the ship’s manoeuvring put it in an appropriate position. At 07.50, Lamotte-Picquet fired a final salvo of torpedoes at 16,400 yards (15000 m), but then lost sight of Thonburi behind an island from which the Thai vessel was not seen to emerge.

At 08.40, Bérenger ordered the Vichy French squadron to head for home, but this coincided with the start of the expected Thai air attacks. Thai warplanes dropped several bombs close to Lamotte-Picquet and scored one direct hit with a bomb that did not, however, detonate. Lamotte-Picquet's 75-mm (2.95-in) anti-aircraft guns put up a vigorous barrage and further attacks were not pressed home. The final attack occurred at 09.40, after which the French squadron returned to Saigon.

Behind it, the Vichy French squadron left a scene of total devastation. Thonburi was heavily damaged and grounded on a sandbar in the mouth of the Chanthaburi river, with about 20 dead. The Thai naval transport Chang arrived at Ko Chang shortly after the French departed and took Thonburi in tow, before deliberately running her aground once more in Laem Ngop, where the ship capsized in the shallow waters.

The torpedo boat Chonburi was sunk with a loss of two men, as was Songkhla with 14 dead. The survivors were rescued by the torpedo boat Rayong, the minelayer Nhong Sarhai and the fishery protection vessel Thiew Uthok. These three ships, which had been sheltering to the north of Ko Chang, wisely chose not to break cover and were not spotted by the French. 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.

The French were given no time to capitalise on their success. Fearing that any further action by the French might turn back the Thai invasion, the Japanese government offered to mediate a settlement, and this ultimately confirmed the Thai annexations of the territory they had lost to France late in the 19^th century. 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 Malaya from the British. At the end of World War II, Thailand was forced to return all its territorial gains to French Indo-China.

Thonburi was later raised, repaired in Japan and used as a training ship until she was decommissioned in 1959.

During the post-action investigations, the Royal Thai navy claimed, on the basis of statements by Thai sailors and the fisherman around Ko Chang and merchantmen in Saigon, that heavy damage had been inflicted on Lamotte-Picquet and her squadron, but these claims are not mentioned in any Vichy French documentation or in the ships' logs.