Dress uniform

Dress uniform (often referred to as Full Dress Uniform, to distinguish it from Mess Dress, and from semi-formal uniforms, such as the British Army's Service Dress), is the most formal military uniform, typically worn at ceremonies, official receptions, and other special occasions; with order insignias and full size medals. The uniform design may be distinct to a service (Marines, Army, Navy, Air Force, etc.), or to a Regiment or Branch of Service. Although they are often brightly colored, and adorned with ornaments (gold braid, lanyards, etc.), most originated as practical uniforms that, with the adoption of even more practical uniforms, have been relegated to ceremonial functions.
Although many services use the term dress generically for uniforms, allowing it to refer to more modern combat uniforms, with suitable modifiers (eg., the British Army's obsolete Battle Dress (BD), and the US Army's obsolete Battle Dress Uniform (BDU), the term Dress Uniform, without a prefixed modifier, is always assumed to refer to the full, ceremonial dress.

United Kingdom
British Army

A parade by officer cadets at the Royal Military Academy Sandhurst
Most of the various uniforms worn by the British Army today, were, historically, combat uniforms. At the start of the 19th century, British Army Regiments of Foot, trained to fight in the manner dictated by a weapon (the musket) which demanded close proximity to the target, were not concerned with camouflage, and wore red coats (scarlet for officers and sergeants). The British infantry literally was a thin red line. Rifle regiments, fighting as skirmishers, and equipped with rifles, were more concerned with camouflage however, and wore dark green uniforms. Light Infantry regiments were also trained as skirmishers but wore red uniforms with green shakos. Whereas the infantry generally wore polished brass buttons and white carrying equipment, the Rifles wore black.
Heavy dragoons and Royal Engineers wore red (or later scarlet) coats. Most of the remainder of the British Army, however, including the Royal Regiment of Artillery, hussars, all but one Lancer regiment, and various support elements wore dark blue uniforms. These varied greatly in detail according to the arm of service or in many cases the individual regiment. From the Crimea War on, a narrow red stripe (piping) down the outside of each trouser leg was common to all red coated infantry units. Cavalry however wore stripes of regimental colour (white, yellow, blue/grey etc)on their riding breeches. Scottish Highland regiments did not wear trousers, favouring the kilt, and Scottish Lowland regiments adopted tartan trews. All Scottish regiments wore doublets of distinctive cut instead of the tunics of English, Irish and Welsh units.

Two Warrant Officers of the Bermuda Regiment in Number 1 Dress. Their uniforms are distinguished from the British Army's by the red cuffs and collars, but are only worn in exceptional situations in winter months by certain soldiers.[1]
Beginning with the Second Anglo-Afghan War of 1878, the British Army began adopting light khaki uniforms for Tropical service. This innovation arose from experience fighting irregular forces in India, say during the Indian Mutiny and Africa during the Anglo-Zulu War, the invention of smokeless gunpowder and the increasing effectiveness and usage of rifles. In 1902 a darker shade of Service Dress (SD) was adopted for field and ordinary use in Britain itself. The scarlet, blue and rifle green uniforms were retained for wear as full dress on parade and walking-out dress when off duty. When khaki web carrying equipment was introduced, the earlier, white or black leather carrying equipment was reduced to just the belt (and sometimes a bayonet frog), for wear with the dress uniform. As with the earlier uniforms, the officers' uniforms differed in quality and detail from those worn by the Other Ranks. Officers purchased their own dress uniforms from regimentally approved Saville Row tailors while other ranks were issued all orders of dress from government stocks.
With the outbreak of World War I in August 1914 all full dress and other coloured uniforms ceased to be worn by the British Army. After 1919 they were restored to the Household Cavalry and Foot Guards for ceremonial purposes but not to the bulk of the army. Officers were authorised to wear full dress for certain special occasions such as Court levees (formal presentations to the Monarch) and it was customary to wear these uniforms at social functions such as weddings. By 1928 bands were wearing full dress on occasions where they were not parading with the remainder of the regiment (who had only khaki service dress). The pre-1914 dress uniforms were still held in store and occasionally reappeared for historic displays. However there was no serious attempt to make them general issue again, primarily for reasons of expense. When (khaki) Battle Dress (BD) uniforms, which had a short blouse instead of a tunic, were adopted immediately prior to the Second World War, the older khaki Service Dress became a smart uniform for wear on the streets, and on moderately formal occasions.
After World War II the coloured, full dress uniforms were again reintroduced for ceremonial occasions by the Brigade of Guards and to a limited extent by regimental bands. Officers (and later senior non-commissioned officers) resumed wearing mess uniforms in traditional colours from about 1956 on. These are still worn, although regimental amalgamations have led to numerous changes from the pre-war models.
The BD uniform was eventually replaced in 1961 by green, cotton combat uniforms. After World War II the design of the Other Ranks' BD blouses had been modified for wearing collared shirts with ties (like the officers' pattern), and was used for a time, around the barracks, but eventually disposed of completely.

The red tunic uniforms worn by British infantry at Rorke's Drift painted by Alphonse de Neuville would soon be replaced in the field by khaki, and relegated to ceremonial use.
With the limited exceptions (Guards, bands, the Essex Yeomanry, The Rifles and a few others) noted, the unique regimental full dress uniforms finally disappeared after 1939. In most regiments they were replaced by a generic dark blue uniform known as No 1 Dress. This dated back to plain "patrol" uniforms worn by officers prior to 1914 as an informal "undress" uniform. An early version had been worn by some units in the 1937 Coronation of King George VI but had not been made general issue at the time. In the form adopted after World War II, most regiments were distinguished only by coloured piping on the shoulder straps, coloured hat bands, buttons and badges. However Scottish regiments retained their kilts or trews as well as the distinctive doublets (in "piper green" or dark blue) of the former scarlet uniform. Rifles had all dark green uniforms and cavalry retained a number of special features such as the crimson trousers of the 11th Hussars or the quartered caps of lancer regiments. A white, lightweight tunic (No 3 Dress) was also authorised for use in the Tropics, or during the summer months in warmer temperate climates (such as Bermuda). The blue "home service" helmets were not worn as part of the No 1 dress uniform, except by members of some bands or corps of drums which retained their old full dress uniforms, at regimental expense. English Rifle regiments were amalgamated into the Royal Green Jackets, which continued to wear a dark green dress uniform, and black buttons and belts. Recent changes have brought the Royal Green Jackets and The Light Infantry together into a single regiment The Rifles, which continues to wear dark green.
Berets were introduced initially into the Royal Tank Corps in the First World War and their use became more widespread in the British Army during and after the Second World War to replace side caps for wear with combat uniforms when protective headgear was not being worn. Originally, khaki was the standard colour for all units, but specialist units adopted coloured berets to distinguish themselves. For example Airborne forces adopted a maroon or red beret. This has since been adopted by many other parachute units around the world. The Commandos adopted a green beret. The Special Air Service (SAS) initially adopted a white beret quickly changing this to a beige or sand coloured one. From 1944 they wore the Maroon Airborne forces beret but the beige beret was re-adopted following the re-formation of the Regular SAS in Malaya.

Changing of the Guard, Buckingham Palace
Khaki was replaced as a generic colour for berets after the war by dark blue, and this is the colour worn by those units not authorised to use a distinctively coloured beret.
Berets fall mostly outside the scope of this article as a peaked cap, with a coloured hat band, is intended to be worn with the No 1 Dress uniform, berets are the most common form of headdress seen with other orders of dress and are worn in No1 and 2 dress by some Regiments and Corps (For a full list see British Army Uniforms). A khaki, peaked cap may also be worn by officers in some units with the No 2 khaki service dress.
The blue or green No 1 Dress was never universally adopted after its initial introduction in 1947. The reason was mainly one of economy, although it was sometimes criticised as being too similar to police and other civilian uniforms - lacking the immediately recognisable military status of both scarlet and khaki. Khaki No 2 dress being the most usual order of dress for parades and formal occasions.
As noted above, the practice of issuing other ranks in line regiments with full sets of both service dress and dress uniforms effectively ended in 1914 and was never completely returned to. Today, with the exceptions noted above, full dress or No 1 Dress uniforms are only held in limited quantities as common stock, and issued only to detachments on occasional special ceremonial occasions. Practices do however vary between units and traditional items of uniform are more likely to appear where tradition is particularly strong. The Royal Military Academy Sandhurst holds dark blue No 1 dress uniforms for the use of its cadets and the Royal Military Police retain this order of dress for general issue.

U.S. Marine Corps

The Marine Corps has a dress blue uniform, in addition to their green service uniform which is part of a long line of historical Marine Corps uniforms dating back to the American Revolution. The most formal of a Marine's uniforms outside of the elaborate evening dress uniforms of officers and senior enlisted, it is often referred to as "Dress Blues", due to its color (as distinguished from the green and khaki service uniforms), and can be worn in many forms. It is the only uniform of the United States military to use all of the colors of the nation's flag.
Dress Blue "A" has a long sleeve choker-collar midnight blue outer blouse, white barracks cover, with all medals and ribbons. Enlisted coats have a red trim and more buttons down the middle of the coat than officers.
Dress Blue "B" is the same as "A", but ribbons and marksmanship badges are worn instead of medals. Dress Blue "A" (with medals worn) is strictly reserved for official ceremonies, while Dress Blue "B" may be worn on leave or liberty.
Dress Blue "C" is the dress blue uniform worn with the long sleeve khaki shirt (without coat). Ribbons and badges may be worn.
Dress Blue "D" is the dress blue uniform worn with the short sleeve khaki shirt (without coat). Ribbons and badges may be worn
All the blue uniforms have the same trousers, cover, and black shoes. Officers, Staff Noncommissioned Officers, and Noncommissioned Officers wear blood stripes on their trousers. Blood stripes are 1.25" in width for NCOs and SNCOs, 1.5" for officers. General officers wear dark blue trousers in the same color as the coat, while all other ranks wear medium/sky-blue trousers.
A sword may be worn when the individual is in command of troops in formation—the Mameluke sword for officers, the NCO sword for NCOs and SNCOs. The sword is worn with the Sam Browne belt; the shoulder strap is worn only by officers with the blue coat. The Marine Corps is currently the only branch of the United States military which regularly allows enlisted members to carry a sword.
Prior to 1998, certain ceremonial Marine units, such as the Silent Drill Platoon, wore a blue/white dress uniform in which white trousers were substituted for blue while performing ceremonial functions. The blue/white version is now an authorized summer uniform for officers and SNCOs.
Another uniform, the obsolete Dress White uniform, was a white version of the standard dress coat and trousers, and resembled the Navy's Officer/CPO dress whites. No blood stripes were authorized, white shoes were worn, and SNCOs wore white and gold rank insignia. This uniform was superseded by the Blue/White Dress uniform in 2000.