The War Hero achievement is a high score based, not an accumulative achievement.
The progress bar on the achievement screen will show the highest Victory Point score you have reachedsince you began playing.
So in other words, the Victory Point count in the achievement won’t decrease if you lose battles, and will only increase once you’ve beaten your old high score.
For example, if your current Victory Point score in your base shows 101 but the highest you ever reached so far was 105, the progress bar in the achievement screen will show 105. It will only update once you you beat that high score and you reach, e.g. 106. You will complete the achievement once you reach the target Victory Point score of the given stage.