If u use the boosts and u then join the placement phase the boosts will be there after it has expired but in attack the attack phase it will expire normally I did this today me that I am at the bottom of my Alliance had the strongest defenders but the boost must still be a active at the time that you join the war ok
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you can use boosts while you are doing AQ and AW for your attackers though. it goes away after the normal amount of time.
Thank you.
A few months ago they did, but it was a glitch/bug, it was fixed and now it 100% has no effect