River

The only way I could see this work is if there was a cool-down and the surrendering nation couldn't declare offensive wars for a short time period after. Hell, you could even make it only usable if the attackers army value was significantly higher than the defender or vice versa. Isn't that just pointless then? Lose 4% of your infra/loot and just clear a warslot for someone else. At a certain point it's more viable to just take the beating.