Ali al-Akbar b. al-Husayn

According to famous narrations, the first martyr from Ahlul Bayt (AS) in Karbala was Ali Akbar (AS). Maula Abbas (AS) was martyred when he went to bring water for the children of Imam Husain (AS) .Ali Asghar (AS) was martyred later on.

Wassalam.

Majority of our great Shia Ulama say that Imam Zainul Abudeen (AS) was elder than Hazrat Ali Akbar (AS). This is stated by:

1. Shaikh Al-Tousi in Kitab Al-Ghaibah based on an authentic narration frim Al-Fudhail Ibn Yasaar from Imam Muhammad Al-Baqir (AS). 
2. Shaikh Al-Mufeed in his book Al-Irshad.

3. Shaikh Al-Tabrasi in Taaj Al-Mawaaleed and also in his book known as E’laam Al-Wara.

4. Abul Qasim Al-Kodinar in Al-Estighathah.

5. Shaikh Sadouq in Al-Amaali.

6. Al-Qadhi Al-Nu’man Al-Maghribi in Sharh Al-Akhbaar who said that it is opinion of Shia Ulama in general.

7. Allamah Al-Hilli in Al-Mustajaad Minal Irshad.

8. Ibn Haatam Al-A’aamili in Al-Durr Al-Nadheem.

9. Ibn Tawoos in Rabee3 Al-Shia.

Many Sunni historians and Ulama claimed that Hazrat Ali Akbar (AS) is elder than Imam Zainul Abideen (AS) according to their own narrations. 
Few of our Shia Ulama like Ibn Idrees Al-Hilli in his book Al-Saraa'ir and Ibn Shahr Aashoob in Al-Manaaqib, and Inb Jareer ibn Rustom in Dalaa'il Al-Imamah, and Shaheed Al-Thani in Al-Doroos had this opinion that Hazrat Ali Akbar (AS) is the elder.

Wassalam.

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.

Wassalam.