Maybe after the 50 Aes Sedai are beaten off at the Black Tower. The Asha'man and Taim will get anxious to attack the White Tower. They will go against his orders and have kind of a mutiny. In that they will go ahead and attack the white tower. Bonus twist: the rebel Aes Sedai will probably just have regained the white tower in a battle or the rebels aes sedai will arrive while the Asha'man are attacking the tower and take them by surprise thus killing most of the asha'man.