An old-fashioned term for this is [***standard-bearer***][1]. It is now more widely used as an idiom for someone who represents a cause of any sort, but the original meaning was the person who carried an army's *standard* or flag. From Oxford Dictionaries:

> 1. A soldier who is responsible for carrying the distinctive flag of a unit, regiment, or army.  
> 1.1 A leading figure in a cause or movement.

This is from the now-rare meaning of [*standard*][2]

> 4. A military or ceremonial flag carried on a pole or hoisted on a rope.

I don't know how widely-understood the literal meaning of the term is, but I would think that military history buffs would know it.

  [1]: https://en.oxforddictionaries.com/definition/us/standard-bearer
  [2]: https://en.oxforddictionaries.com/definition/us/standard