That new players heroes and troop are probably less strong than the ex-player. Also, new player must have loose his last wars. So he bring less points to your war score, thus the decrease.
War Score are calculated based on the most powerful heroes and troops of each alliance member participating in the wars, adjusted by the past performance in wars. Although, @Nimppy you are correct that the score could have been reduced because the new member was not as strong as the previous, I do not think so that his performance on this previous alliance is counted; the only thing taken into consideration is @Bukefal alliance performance on previous wars
No they changed this to tie performance to player rather than alliance to stop the win then form new alliance method people were using to game the system
It was a while ago. The way it used to work was win 5 wars open chest, then fold alliance and move to new one where there is no win history and get easier opponents.
@Kilted is right. War score is attached to the players not the alliance. Wherever you go, you take your war score with you. That’s to avoid war score manipulation by changing alliances.
Thats nice to know, I noted that there were a lot of alliance members on opposing teams with less than a couple days membership at times. On the other hand, there are a few alliance members who jump to brother alliance to help with titans, and others who merc when they can. They would have minimal time in the alliance after rejoining.