The British Army certainly provided the bulk of the military capability for much of the period of imperial history. Keeping track of the various regiments and units can be a little tricky though as the army evolved, changed and responded to new challenges over time. The first major innovation was in 1694 during the Nine Years War when arguments over which regiment had precedence over the other regiments was resolved by William of Orange who issued an official order of precdence. This was amended in 1713 and again in 1715 where the principle of English regiments being ordered according to their date of raising was combined with other non-English regiments being ordered according to the date of their accession into the English Army establishment. Increasingly regiments were referred to by this order of precedence rather than their colonel's name which was increasingly confusing. It was not until 1751 though that a Royal Warrant was issued to mandate referral of the units by their Regimental number.
The next major innovation was introduced in 1782 as a result of the relative poor showing of the British Army in the American War of Independence and the way there were perceived by the populations that they were supposed to be protecting. It was decided that the numbered regiments should be associated with specific counties and geographical areas in order to foster greater allegiance and identification between regiments and the home country. This innovation was more for the benefit of the local population who it was hoped would feel better disposed towards a unit with a local identification. Those volunteering for service on the other hand could often find themselves being allocated to any regiment which had a shortage of personnel and might be based many miles away from their home. Furthermore, regiments often campainged overseas or went on garrison duties around the Empire for years on end.
The full 'Territorialisation' concept was further enshrined in the next major set of reforms in the wake of the poor showing of the British Army in the Crimean War from 1854 to 1856. The organisational and structural failings of this war led to a Royal Commission into the organisation of the Army which further led to both the Cardwell and Childers Reforms culminating in the major 1881 reorganisation. Effectively, the numbering system was dropped totally (although the order of precedence concept remained in place) and localisation was enhanced for recruits making it more likely that they might spend some time in a region that was local to them. This territorial innovation also allowed for the concept of a regiment having two (or more) battalions on service with one likely to be based in the home depot whilst the other was off on foreign service usually somewhere in the British Empire. This county regiment would also provide training, facilities and expertise to local militia and yeomanry regiments. The authorities had been impressed by the mass mobilisation of the Prussian armies in the Franco-Prussian war and sought to find a way that they could raise large numbers of soldiers without having to resort to conscription as most of Europe used. This territorial concept was their answer with the regiments being able to expand in size with additional battalions in time of war. This was very much the system in place during the First World War and was flexible enough to lead to a massive expansion although the demands of the war still saw the need to resort to conscription despite Britain's long antipathy to forcing people to fight.
1922 saw the next major series of reforms in the wake of the First World War. The army had expanded massively to fight this war but victory, changing technology, the extent of the losses and a hope that no major war would ever have to be undertaken again saw a radical downsizing of the British Army with many regiments being amalgamated in the process. Cavalry regiments were particularly vulnerable given that the First World War revealed serious limitations of the use of the horse on the modern battlefield. Five regiments were also lost to the British Army with the creation of the Irish Free State.
Mechanisation and rearmament in the 1930s saw the creation of new formations as well as many cavalry formations being re-equipped and renamed.
The British Army may have been the pre-eminent armed force during the British Empire but the Indian Army provided a huge infusion of personnel over the years. Originally it had been the private army of the Honourable East India Company but after the Indian Mutiny of 1857 and 1858 it was radically reorganised and rearmed. For a while at least, Indian regiments were not allowed the latest weaponary for instance and usually were one generation behind equivalent British regiments in terms of equipment and technology. Having said that, its sheer size provided the British Empire with an alternative source of military power especially in Asia , the Middle East and in Eastern Africa. The fact that the Indian Army was self-funded from the Government of India also helped convince financially minded British governments of the benefits of using Indian soldiers over British soldiers on several occasions. In 1895 the three separate Presidency armies of Bengal, Madras and Bombay were amalgamated into a single Indian Army. This was formalised in the 1903 reorganisation.
Other colonies were also pressed to rely less on costly British troops and provide more of their own troops. From their earliest days, colonies often raised their own militia units to complement the British regular army but over time they were pressed to raise and maintain their own full time regiments. The 1860s and 1870s seemed a particularly important transition period as governments in Australia and New Zealand for instance were informed that they were expected to raise their own taxes to pay for their own land security. It was understood that the Royal Navy would still be available and that additional imperial troops could be provided in emergencies but the principle was increasingly that colonies provided for their own protection. The extent of the growth of colonial forces was revealed in the Boer War and the World Wars when the colonies and dominions provided considerable military support to the British establishment. Increasingly, Australian, Canadian and South African officers rose to the very top military roles at the very highest levels of imperial defence.
Unfortunately, keeping straight the names of the various regiments in the various armies who played a role in the defence of the British Empire can be a tough undertaking. We have attempted to put together a matrix of the various formations at various stages during the British Empire. You can click on their names for more detailed regimental histories.