Sayyed Mohammad Al-Musawi, Sayyed Mohammad al-Musawi is originally from Iraq and heads up the World Ahlul Bayt Islamic League in London. Other than being involved in various humanitarian projects, he frequently responds to... Answer updated 2 months ago

The claimed participation of Imam Hasan and Imam Husain in any war of expansion is not authentic and does not have a trusted chain of narrators and it was never narrated in any authentic Shia reference among all of the narrations of the life of imam Hasan and Imam Husain (AS). It was mentioned in two Sunni books ( Tareekh Al-Tabari and Al-Ibrar by ibn Khaldoon who died in 808 Hijri, with out authentic chain of narrators. That is why, we can not trust such claim. We also know that the infallible can never be under leadership of a sinner. 
Expansionistic wars were motivated by rulers who wanted more wealth and more control under the slogan of Islam with out permission from the real leader of Islam after the Prophet who is Imam Ali (AS).

Wassalam.