Coast Defences at Diego Garcia

Diego Garcia was one of several ‘secret anchorages’ in the Indian Ocean that might be put into use in the event of war with Japan.  These defended bases were intended to provide refuelling stops for use by the Royal Navy when sending naval reinforcements to Singapore.  The most important for these bases were: the Seychelles; Diego Garcia in the Chagos Islands; Addu Atoll in the Maldives; and Nancowry in the Nicobar Islands.  By the summer of 1941, plans were being made to install coast defence artillery and other facilities on these islands.  In August, the Admiralty confirmed its intention to have two 6-inch coast defence guns installed at Diego Garcia.  India Command was charged with raising the coast battery personnel to man these guns and those to be installed on the other Indian Ocean islands.[1]

On 5th September 1941, the Governor of Mauritius having been previously consulted, the Officer Commanding Troops, Mauritius was informed by cable of the plan to install coast defence guns at Diego Garcia.  He was asked for details of what help he could provide in raising the personnel for the gun battery and for a garrison company.  The O.C. Troops, Mauritius, replied on 16th September, that there were no suitable British officers available to man the gun battery at Diego Garcia.  In addition, the other ranks needed could only be found following the implementation of compulsory military service.  It was thought that these men would then be available to move to Diego Garcia five months after they had been conscripted.  The garrison company could be raised by increasing the establishment of the Mauritius Territorial Force and recruiting the personnel from the local inhabitants of Mauritius.  It was thought that the new garrison company would require one month to raise and three months to train.  As soon as accommodation could be built at Diego Garcia, the company could deploy there and complete training.  Additional officers and men would be required to provide reliefs.  On 20th September 1941, a British officer and ten other ranks of the Mauritius Territorial Force were sent to Diego Garcia for temporary guard duty.[2] 

India Command confirmed on 29th September 1941, that it could provide four British officers for the gun battery at Diego Garcia.  However, the officers would not be available before 1st November at the earliest.  On 12th October 1941, it was confirmed that Mauritius was to provide the coast battery and garrison company for Diego Garcia and that the new units would not be available before January 1942.  India Command agreed to provide a section of artisan works engineers and a gunner officer to supervise installation and construction of facilities, together with the necessary equipment and materials.  India also agreed to provide a detachment of medical staff.  Delays in the introduction of conscription on Mauritius resulted in revised estimates of readiness for the units being raised: for the garrison company, early February 1942; for the gun battery, the end of February 1942. 

By September 1941, it had been decided that the coast defence guns would be installed and temporarily manned by a detachment of Royal Marines provided by the Mobile Naval Base Defence Organisation, then in Egypt and preparing to leave for the Indian Ocean.  The Marines detailed to complete the work at Diego Garcia were known as Force ‘Shortcut’.  This detachment, along with Force ‘Piledriver’, went first to Addu Atoll where several coast defence guns were installed.  In late November 1941, the support ship Clan Forbes carried Force ‘Shortcut’ to Diego Garcia in the Chagos Archipelago.  The ship arrived at 0800 on 28th November 1941, to find H.M.S. Laomedon and H.M.I.S. Clive already at anchor.  A reconnaissance of the channel and landing place was conducted later that day.  The following day, the unloading of stores and equipment began.  The accompanying section of the 15th Artizan Works Company, I.E. began construction of a camp.  On 30th November, the first of two gun pedestals was unloaded from the Clan Forbes and taken to the gun site at Eclipse Point.  Work also began to establish a water supply as quickly as possible.  On 1st December, the second gun pedestal was unloaded and taken to the gun site.  The first of the 6-inch guns was unloaded the next day, followed by the second on 3rd December.  Stores were reloaded on board the Clan Forbes while the Laomedon continued to unload.  The personnel re-embarked on board the Clan Forbes on 4th December except for one section of the Landing Company, two M.L.C. crews and men from the Workshop Company who embarked upon the Laomedon to continue unloading.  At 1900 that evening, the Clan Forbes sailed for Colombo.[3] 

In January 1942, the Royal Marines returned to Diego Garcia to complete the installation of the guns.  They arrived on the morning of 15th January 1942 and unloading began that same day.  Equipment for mounting the guns was unloaded the following day and one gun and cradle, landed previously in late November-early December 1941, were transported to the gun site at Eclipse Point.  This gun was mounted on 18th January and the second the following morning.  Meanwhile construction of a camp and other facilities continued.  On 21st January, the personnel of the A.M.T.B. Battery, R.M. landed to man the battery at Eclipse Point.  The first proof rounds were fired the next day and 50 rounds of ammunition were landed and stored in temporary magazines at the gun site.  On 27th January, H.M.S. Carthage and the Royal Fleet Auxiliary Changte arrived carrying the permanent battery personnel and their stores.  On board the Carthage were the Royal Artillery and Mauritius Territorial Force personnel who were to man and guard the guns installed by the Royal Marines.  These were the “X” Mauritius Coast Battery and a Mauritius Garrison Company.  These disembarked upon arrival and the personnel of the A.M.T.B. Battery re-embarked upon the Clan Forbes and the Carthage sailed that same day.  Unloading and construction work continued, including the establishment of a tented hospital and other facilities.  H.M.I.S. Clive and R.F.A. Changte sailed late afternoon of 30th January.[4]

Construction work continued into the first weeks of February 1942.  The Clan Forbes provided the accommodation for the Royal Marines throughout their stay at Diego Garcia.  As the ship prepared to sail once again, a small detachment disembarked on 18th February to continue the work and the Clan Forbes sailed the next day for Addu Atoll to collect additional stores.  The ship returned to Diego Garcia on 26th February and immediately began to unload the stores.  Additional Royal Marines disembarked to help speed up the final construction of the water supply system.  Working with the Indian Artizan Company, construction work continued.  All Force ‘Shortcut’ personnel and equipment re-embarked on the Clan Forbes between the afternoon of 12th March and the afternoon of 13th March.  The ship sailed that afternoon for Colombo.[5]

In the meantime, consideration had been given to supplementing the defences at Diego Garcia.  Following preliminary construction and installation work at Diego Garcia, M.N.B.D.O. recommended on 28th January 1942, that the scale of artillery defences was inadequate should the anchorage be used by cruisers or large ships.  It was felt that at least one additional 6-inch gun battery would be required to protect the anchorage from bombardment from the east.  It was further suggested, that close defence equipment should be installed at Eclipse Point – namely anti-motor torpedo boat guns and searchlights.  If a seaborne air attack were to be expected, then anti-aircraft defences would also need to be installed.  In response to the M.N.B.D.O. report, on 7th February 1942 a cable was sent by the C-in-C East Indies.  In this, while it was accepted that the general sense of the M.N.B.D.O. report was correct, it was proposed that the limitations of the then present defences had to be accepted for the time being.  Given the rapidly changing situation with the war against Japan, a war that was going badly, it was felt that it was not possible to set down the use to which the anchorage at Diego Garcia might be put to.  While no decision to install further defences was proposed, it was put forward that it would be sensible to begin raising and training personnel for an additional 6-inch battery, should one be needed in the future.[6]

The February 1942 Defence Plan for Diego Garcia made several assumptions of the possible future use of the naval base by larger ships with heavy draught.  However, it was thought that the existing channel to the anchorage was suitable only for cruisers and small aircraft carriers.  These assumptions would have to be tested but in the meantime two outline plans were put forward, assuming the possible future development of the base to support larger ships.  Both plans identified the requirement for significant numbers of heavy and light anti-aircraft guns and Plan ‘B’ identified the need to install two batteries of modern 5.25-inch coast defence guns.[7]

A reconnaissance was made of the island in March and April 1942, with a view to establishing additional defence requirements.  The proposals confirmed much of what had been assumed in February 1942.  The Reconnaissance Report of April 1942 included the following key recommendations for defence of an expanded naval base:

-       provision of range-finding equipment for the existing 6-inch coast defence battery at Eclipse Point,
-       subsequent replacement of the 6-inch coast defence guns with two, three-gun batteries of modern 5.25-inch guns at Eclipse Point and at Horsburgh Point,
-       installation of two, two-gun Bofors anti-motor torpedo boat batteries, one at Barton Point and one on West Island,
-       installation of 16 heavy and 16 light anti-aircraft guns,
-       the infantry garrison to be increased from a single company to a full battalion. [8]

However, as the war progressed, the threat of Japanese attack to Diego Garcia receded and the purpose of the naval base was limited to the occasional refuelling of light craft and to support flying boat operations.  It was decided not to proceed with the installation of the proposed additional artillery defences and the two 6-inch guns at Eclipse Point remained as the island’s only coast defences. 

In September 1942, with the defence of Diego Garcia now entirely the responsibility of Indian Command, the Mauritian troops were themselves relieved by 12th Indian Coast Battery and a company of the 25th Garrison Battalion, 4th Bombay Grenadiers.  The Indian personnel of the 12th Coast Battery and the company of the 25th Garrison Battalion, 4th Bombay Grenadiers were withdrawn in the early months of 1944.[9]

The two coast defence guns remain on the island to this day, at what is now known as Cannon Point.[10] 

24 June 2021