The Royal Garrison Artillery (RGA) was formed in 1899 as a distinct arm of the British Army's Royal Regiment of Artillery serving alongside the other two arms of the Regiment, the Royal Field Artillery (RFA) and the Royal Horse Artillery (RHA). The RGA were the 'technical' branch of the Royal Artillery who were responsible for much of the professionalisation of technical gunnery that was to occur during the First World War. It was originally established to man the guns of the British Empire's forts and fortresses, including coastal artillery batteries, the heavy gun batteries attached to each infantry division and the guns of the siege artillery. The RGA was amalgamated with the RFA in 1924, from which time the only two arms within the Royal Regiment of Artillery have been the Royal Artillery and the Royal Horse Artillery.
Organisation
The Royal Garrison Artillery came into existence as a separate entity when existing coastal defence, mountain, siege and heavy batteries of the Royal Artillery were amalgamated into a new sub-branch. A royal warrant provided that from 1 June 1899: "... the mounted and dismounted branches of the Royal Regiment of Artillery shall be separated into two corps... to be named respectively (a) the Royal Horse Artillery and the Royal Field Artillery: (b) the Royal Garrison Artillery."
The Royal Regiment of Artillery, thenceforth, was divided into four branches. Other than mounted or unmounted dress, the obvious distinction in uniform was by the shoulder title badges: RA (for the branch tasked with managing ammunition dumps and supply to units in the field); RGA; RFA; and RHA.
With the increasing importance of artillery defences by the mid-Nineteenth century (and the usual reluctance of the British Government to fund an expansion of the regular military forces), a military reserve artillery force became a pressing concern to aid in maintaining the fixed defensive batteries. Through the Eighteenth Century, and up to the end of the Napoleonic Wars and the American War of 1812, the military reserve forces that supported (but were not part of either) the British Army and the Board of Ordnance Military Corps (which included the Royal Artillery, the Royal Engineers, and the Royal Sappers and Miners) included the Militia, which was normally an all-infantry force, and the mounted Yeomanry. During wartime, these were supplemented by Volunteer units. Royal Artillery coastal batteries were often brought up to strength with drafts from the British Army or the militia, or by temporarily re-tasking militia units or raising volunteer artillery corps. In Bermuda, from the Seventeenth Century until after the American War of 1812, men with status and the required funds were appointed as Captains of forts (which spared them from any obligation to serve in the Militia), in command of fortified coastal batteries manned by volunteers through peace and war.
In 1852, with fear of an invasion of Britain by France, the reserve forces were re-organised. The Militia, which had become a paper tiger, changed from a conscripted force to one in which recruits voluntarily engaged for a term of service. It also ceased to be an all-infantry force. As the most critical shortage was of garrison artillery, a number of Militia Infantry regiments were re-tasked as Militia Artillery, and new militia units were raised as artillery. The Militia Artillery units, which (like other reserve units) were raised under the Lords-Lieutenants of counties, who appointed officers), and were all tasked with garrison duties at fixed batteries. The invasion scare also led to the re-establishment of the Volunteer Force as a permanent (though only part-time, except when embodied for emergencies) branch of the British military. This force (which differed from the Militia primarily in that its volunteers did not engage for a term of service, and might quit with fourteen days notice, except while embodied) contained a mixture of artillery, engineering and infantry units. Similar militia and Volunteer units were also raised in various British colonies.