A peace deal has to be set to come into effect at some point in the future so that the message can be spread along a very long line of soldiers, some of whom may not have regular radio contact or be in small pockets. Not only do you need time to inform everyone, you also have to set it at a specific time after everyone is informed, so that you can make preperations with that in mind - for example, getting ready to recover stranded wounded after the ceasefire.