If an alliance is unprepared for war then it's their own fault. They either failed in the political side of the game or they lack the skills to manage their alliance effectively, and the game shouldn't be catered to them because of it. Wars in this game are first fought and mostly won on the political field through containing/managing information, getting support, and isolating your target as best you can. If your enemies manage this and prep themselves without you noticing or getting word of it in time, then you deserve to be at a disadvantage that your negligence put you in.
This suggestion would also make it much harder to deal with raiders and rogues. They can be more bold with who they target with little to fear, cause what can you do about it? Beige them with limited damage and let them come back later. Lets go back to when Apeman went nuke rogue on BoC, imagine that with this war system. He would have been able to come in on 5 targets, nuke them, get beiged, build back up and go again. This would have greatly extended the duration he could have dealt major damage to BoC and reduced damage to his own nation considerably.