A war flag is a variant of a national flag

National flag

A national flag is a flag that symbolizes a country. The flag is flown by the government, but usually can also be flown by citizens of the country.Both public and private buildings such as schools and courthouses may fly the national flag...

 for use by the nation's military forces on land. The nautical equivalent is a naval ensign — the battle ensign

Battle ensign

A battle ensign is the name given to a large war flag which is flown on a warship's mast just before going into battle.The flag identified the allegiance of the ship in what could be a very confusing situation, with thick clouds of gunsmoke obscuring the ships in action, hence the large size of...

. Under this strict sense of the term, few nations currently have war flags, most preferring to use instead their state flag

State flag

There are two separate meanings for the term state flag in vexillology – the flag of the government of a sovereign state, and the flag of an individual subnational state.-Government flag:...

 or standard national flag for this purpose.

History

Field sign

Field sign

A field sign is an unofficial differencing mark worn on a combatant's clothing to show the difference between friend and foe or a combatant and a civilian.-Examples:...

s were used in early warfare at least since the Bronze Age

Bronze Age

The Bronze Age is a period characterized by the use of copper and its alloy bronze as the chief hard materials in the manufacture of some implements and weapons. Chronologically, it stands between the Stone Age and Iron Age...

.
The word standard itself is from an Old Frankish

Old Frankish

Old Frankish is an extinct West Germanic language, once spoken by the Franks. It is the parent language of the Franconian languages, of which Dutch and Afrikaans are the most known descendants...

 term for a field sign (not necessarily a flag).
The use of flags as field signs apparently emerges in Asia, during the Iron Age, possibly in either China or India. Early field signs that include, but are not limited to a flag, are also called vexilloid

Vexilloid

"Vexilloid" is a term used tenuously to describe vexillary objects used by countries, organizations, or individuals as a form of representation other than flags. Coined by Whitney Smith in 1958, he defined a vexilliod as:...

 or "flag-like", for example the Roman Eagle standard

Aquila (Roman)

The Aquila was the eagle standard of a Roman legion, carried by a special grade legionary known as an Aquilifer. One eagle standard was carried by each legion.-History:...

 or the dragon standard

Draconarius

For the spider genus in the family Amaurobiidae, see : Draconarius .The draconarius was a type of signiferi who bore a cavalry standard known as a draco in the Roman army.- Name :...

 of the Sarmatians

Sarmatians

The Iron Age Sarmatians were an Iranian people in Classical Antiquity, flourishing from about the 5th century BC to the 4th century AD....

. The Roman Vexillum

Vexillum

The vexillum was a flag-like object used in the Classical Era of the Roman Empire. The word is itself a diminutive for the Latin word, velum, sail, which confirms the historical evidence that vexilla were literally "little sails" i.e. flag-like standards...

 itself is also "flag-like" in the sense that it was suspended from a horizontal crossbar as opposed to a simple flagpole.

Use of simple flags as military ensigns becomes common during the medieval period, developing in parallel with heraldry

Heraldry

Heraldry is the profession, study, or art of creating, granting, and blazoning arms and ruling on questions of rank or protocol, as exercised by an officer of arms. Heraldry comes from Anglo-Norman herald, from the Germanic compound harja-waldaz, "army commander"...

 as a complement to the heraldic device shown on shields. The maritime flag

Maritime flag

A maritime flag is a flag designated for use on ships, boats, and other watercraft. Naval flags are considered important at sea and the rules and regulations for the flying of flags are strictly enforced...

 also develops in the medieval period. The medieval

Muromachi period

The is a division of Japanese history running from approximately 1336 to 1573. The period marks the governance of the Muromachi or Ashikaga shogunate, which was officially established in 1338 by the first Muromachi shogun, Ashikaga Takauji, two years after the brief Kemmu restoration of imperial...

 Japanese Sashimono

Sashimono

Sashimono were small banners historically worn by soldiers in feudal Japan, for identification during battles.-Description:The sashimono poles were attached to the backs of the chest armor by special fittings. Sashimono were worn by common soldiers, known as ashigaru, to elite samurai, and in...

 carried by foot-soldiers are a parallel development.

Some medieval free cities or communes

Medieval commune

Medieval communes in the European Middle Ages had sworn allegiances of mutual defense among the citizens of a town or city. They took many forms, and varied widely in organization and makeup. Communes are first recorded in the late 11th and early 12th centuries, thereafter becoming a widespread...

 did not have coats of arms, and used war flags that were not derived from a coat of arms. Thus, the city of Lucerne

Lucerne

Lucerne is a city in north-central Switzerland, in the German-speaking portion of that country. Lucerne is the capital of the Canton of Lucerne and the capital of the district of the same name. With a population of about 76,200 people, Lucerne is the most populous city in Central Switzerland, and...

used a blue-white flag as a field sign from the mid 13th century, without deriving it from a heraldic shield design.