More specifically, how was war declared? Was there a code of ethics each nation followed to give the other nation time to prepare before an attack after the initial declaration of war? When the war was over, how long did it usually take to get the message out to all of the generals so the fighting would stop? Were there laws to allow messengers (such as the ones who would deliver that message that the war was over) free passage without harm?
I searched on this subreddit and on google and really couldn't find anything about it. I was just thinking about this today during my early modern history class and it raised my curiosity.
Well it greatly depends on how far you want to go back. The Greeks and other empires of the era used foot messengers, such as the messenger who ran the marathon hills and then died. In later years, closer to modern era, pigeons were used extensively because of their innate homing ability. As to ethics, as far as I am aware there really wasn't any. It would make more sense to attack without warning when your goal is conquest. In the era of pigeons, enemy troops would often release Hawks to kill the pigeons so that the message never gets received.
Hope this helps!