Yaun isn’t an empire or republic, it is a Confederacy of independent city-states and their surrounding lands. The members all agree to respect each other’s laws within their own lands, while agreeing to laws that effect the general safety and prosperity of the whole.
Yaun grew out of a mutual defense treaty that is still the core of its existence. Several cities agreed to come to each-other’s defense in the case of an invasion. Eventually, it became useful for the members to station a compliment of troops in each of their allies’ towns. This increased the coordination of military forces, and led to increasing familiarity amongst the nobles of the various cities. Eventually, custom became law, and all the Yaun members agreed to limits on their militaries.
Each state is garrisoned by a small professional army, two thirds of which is made up of local soldiers and one third of which is made up of soldiers from every other member of the Confederacy. So while each home army is the supreme force within its own borders, no state has enough troops to repel an invasion without the help of their allies. Additionally, any potential belligerent is keenly aware of a sizable foreign force within its own borders. This system has led to troops constantly moving throughout the Confederacy, as various states rotate their soldiers to and from domestic duties and postings within other state armies. This has resulted in the trade routes being constantly patrolled, encouraging long distance trade as caravans accompany soldiers travelling between posts.
Yaun has no permanent capital, but instead holds Spring and Fall meetings of state delegates. These meetings are hosted by all members on a rotating basis.