You can get Alliance XP from Wars in 2 ways:First, by capturing alliance war flags. With each flag you earn 3 alliance XP. So in an Alliance War, a player can earn up to 9 AXP per battle and in total 18 AXP by earning all the flags in the War.
Your alliance can also win a huge amount of AXP by winning the war. A fixed amount of 4000 AXP can be earned in total by winning the alliance wars. 
How will AXP distributed among each members of the Alliance?When an alliance wins a war, the 4000 AXP will be distributed equally among the player who have at least earned 1 flag during the war.
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Empires & Allies Game GUIDE