After persuading a Prisoner, you will lose some Prestige if you decide to have them join your Army. If you lose too much Prestige, you will be unable to recruit more Prisoners. However, for each Imperial Army Force you defeat on the battlefield, you will gain some Prestige. Defeating stronger Imperial Army Forces and releasing Prisoners will also increase your Prestige in the eyes of your subjects.
Diplomatic Documents increase Prestige instantly when consumed. You can obtain Diplomatic Documents by Battle Victory, Missions, Events, Clearing Fog, Investigating World Maps, Alliance Shop, Black Market, and SHOP.
Visit Kingdom Hunter's Official Guide for more information about the game.