Allah caused the day of Buath to take place before Allah's Messenger (ﷺ) was sent (as an Apostle) so that when Allah's Messenger (ﷺ) reached Medina, those people had already divided (in different groups) and their chiefs had been killed or wounded. So Allah made that day precede Allah's Messenger (ﷺ) so that they (i.e. the Ansar) might embrace Islam.