The history of army policing in Great Britain dates back to the middle ages. However, the predecessor units to the modern Royal Military Police were only formed in the 1870s and 1880s, merging in 1926 and being granted their 'Royal' prefix in 1946.
Pre-1850
The post of provost marshal has existed since William of Cassingham was appointed by Henry III on 28 May 1241 (the original title was Serjeant of the Peace). The first personal record of a provost marshal dates to 1511. The provost marshal has always had men detached to assist him, an arrangement formalised by the Duke of Wellington in the Peninsular War.
During the Peninsular War from 1809–14, the Duke of Wellington asked for a provost marshal to be appointed to hang looters; by the end of the Peninsular War the provost marshal controlled 24 assistant provost marshals. Members of this Staff Corps of Cavalry were identified by a red scarf tied around the right shoulder; whilst some consider this to have been the origin of the famous 'Red Cap' of the Royal Military Police and its forebears, it was more likely a precursor of the 'MP' armband (and now the Tactical Recognition Flash), which identifies the modern Military Policeman or Policewoman.
Although disbanded in 1814 at the end of the Peninsular War, the Duke of Wellington re-formed the Staff Corps of Cavalry to police the occupying British Army in France following Napoleon's defeat at the Battle of Waterloo, though it was disbanded again in 1818.
1850-1914
In the Crimean War, a Mounted Staff Corps of almost 100 troopers from the constabularies of Ireland and the Metropolitan Police was established to prevent the theft of supplies, and to maintain discipline in camps. This 'Corps' was disbanded with the cessation of hostilities. However, in 1855 a Military Mounted Police was formed to police the new military cantonment at Aldershot, though it was only in 1877 that it was established as a regular or Permanent Corps.
A parallel Military Foot Police (MFP) was formed in 1885 for campaign service in Egypt, though it was established as a Permanent Corps later the same year. The Military Mounted Police first engaged in combat in 1882 at the Battle of Tel el-Kebir. Although technically two independent corps, the two effectively functioned as a single organisation.
1914-1930
In 1914 the Corps of Military Mounted Police and the Corps of Military Foot Police had a total establishment of nearly 5000 men. When the British Expeditionary Force was sent to France in that year, each division had one assistant provost marshal in the rank of major and several NCOs. The provost marshal was a colonel until 1915, and a brigadier-general thenceforward.
During the retreat from Mons the MPs were busy dealing with soldiers who, through exhaustion or the general confusion of battle, had either lost or became detached from their units. By operating stragglers' posts, the MPs were able to return soldiers to their units. These posts were also well placed to pick out deserters and those Absent Without Official Leave (AWOL).
The First World War was the conflict where traffic control became an important function. This was identified particularly after the Battle of Loos, when there was a lot of confusion involving two British divisions. As well as traffic control, the BEF provost units dealt with the maintenance of law and order (i.e. the detection of crime and the arrest of offenders), custody of prisoners of war until handed over to detention facilities, surveillance, control and protection of civilians. The Military Foot Police's and Military Mounted Police's duties extended to policing the Royal Flying Corps until a separate Royal Air Force Police and Royal Air Force Police Special Investigations Branch were set up on the Royal Air Force's formation in 1918.
The work undertaken by MPs was not all carried out behind the lines, and sometimes they came under heavy fire. During this conflict, the Military Police suffered 375 casualties. Sixty-five received the Distinguished Conduct Medal and 260 received the Military Medal. In 1926 the Military Foot Police and Military Mounted Police were fully amalgamated to form the Corps of Military Police (CMP).
1930-1945
FSP, Supplementary Reserve and TA
A Field Security Police (FSP) Wing was formed within the new Corps in 1937. Its personnel wore Lincoln green cap covers and brass shoulder titles on their tunics with the letters "FSP", to distinguish them from the rest of the Corps. They wore the standard CMP cap badge, but unofficially ground down the wording "MILITARY POLICE" from the lower scroll of the badge.