Ali al-Akbar b. al-Husayn
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We believe that every act of the Infallible is based on the complete wisdom which may not be available to us as our thinking is limited.
The wisdom behind the way of fighting the enemies in Karbala can be to avoid immediate collapse of the camp of Imam Husain (AS) looking at huge numbers of the enemies facing small number of believers.
It can be also to give more chances to the army of the enemies to reconsider their stand.
After all, the whole wisdom of that arrangements is with Imam Husain (AS) which aims definitely to serve the noble aims of Islam.