The Union flag has particular significance to the Armed Forces. Flags of contrasting col0urs have been used to show the position of commanders in the field and their armies from early times. The heavy defensive armour of medieval times made recognition symbols essential, and flags or colours were necessary as rallying points for the horse and foot soldiers. One example of the historic importance of colours as the focus of loyalty to the Crown (and to the regiment) in the annual Trooping the Colour ceremony at Horse Guards Parade on the Sovereign's official birthday when The Queen as Colonel-in-Chief of each of the five regiments of Foot Guards takes the salute. As an emblem of 'Her Majesty's Service', the Union flag is the flag of the Army, which unlike the Royal Navy and the Royal Air Force, does not have its own ensign. 

The Union flag as the King's / Queen's Colour was therefore carried into battle, along with their own Regimental Colours, by a number of Army regiments (artillery did not need them, as their rallying point was the right-hand gun; rifle regiments do not carry colours as their original role was as scouts, which required stealth and concealment). British Regimental Colours were carried into battle for the last time in January 1881, when the 58th Foot (now part of the Royal Anglia Regiment) took their Colours into action against the Boers in the Transvaal, South Africa. Today, Infantry battalions have two colours of flags: the Queen's Colour (a Union flag with the title and number of the battalion at the centre surmounted by St Edward's Crown, with the First and Second World War battle honours on both sides), and the Regimental Colour (featuring the regimental badge and battle honours awarded for campaigns other than the two World Wars. Nowadays, the Union flag is also carried in smaller form on the sides of the Army vehicles and worn on uniforms as a distinguishing national emblem. In the Royal Navy. In the Royal Navy, flags and ensigns assumed the same importance as standards and colours in the Army. Until 1864, fleet were organised into White, Red and Blue squadrons, but in that year Queen Victoria ordered that the White Ensign - the red cross of St George with the Union Flag in the top left-hand corner - should be carried by all ships of the Royal Navy. The Naval Reserves and the Mercantile Marine were to fly the Blue and Red Ensigns respectively. There are also Queen's Colours for the Royal Navy in each of the present-day naval commands, based on the White Ensign, which are carried on suitable ceremonial occasions by naval colour parties. King's or Queen's Colours have also been presented to the other Services. Colours were presented to the Royal Marines (formed as the Admiral's Regiment in 1664) by the Duke of Clarence (the future William IV) in 1827; today there are Queen's and regimental colours for each Royal Marine Commando. In 1947, George VI approved the award of King's Colours to the Royal Air Force for carrying on ceremonial occasions; King's or Queen's Colours have subsequently been presented, amongst others, to the Royal Air Force in the United Kingdom in 1951 and the Royal Auxiliary Air Force in 1989.