I feel like not much will change with this, except the winning nation will have to have 1-2 more immense triumphs. IMO the actual issue causing wars to end quickly tend to be because of double buying by the attackers, strategic planning with the other people in the defender's slots, and inactivity of the defender.
All 3 of these are user-generated and are just smart ways of playing the game. Adding resistance during a war wouldn't really change anything except make the attackers attack 1-2 more times(look at fortify and how useless that was).
I will say this if we were to removal double buying capabilities and then add this mechanic that could be interesting. There would still be some questions that would need to be answered, but that seems more of a reasonable answer to make wars more competitive than just forcing the losing nation to take more hits.