Step 1: prevent nation transfer into the "Strongest" nation
Step 2: give free nation transfer into the "weakest" nation
Step 3: put in some kind of incentive for members of the weakest nation to level up or become stronger but the handicap needs to be limited to prevent abuse.
Example Handicaps:
-2x exp for all maps belonging to the weakest nation.
-2x HP for all war stones for the weakest nation
-The strongest nation starts war with 2 dead stones in GG
-Temporary Dark Bear/Cyclopes /Fatal spawn on a map inside of the weakest nation (this allows players of the weakest nation nation to level safely without getting PKed by the other nations so they can level up and be stronger in war)
Once any of those handicaps are applied, they cannot be used again for 1 month so that nations don't abuse these handicaps by losing war on purpose.