A tradition
In peacetime, the ruler grows their hair long. In war, they cut it short.
A ruler with long hair is held in great esteem, for defending the peace.
The traditional declaration of war is for the ruler to send their cut-off hair to the enemy ruler. The statement carries greater weight the longer the hair: to receive long hair says that you have angered one who is slow to anger, that you have incurred a wrath not easily woken.I also like the aspect that you have to send over a literal part of your physical self, that you have to cut off something that is so highly connected to prestige and esteem in many cultures and lay it down at the feet of you opponent.


