I want to know the details (like why the war started, death of Sahabah, end of the war and outcome) about the wars that took place between 'Ali and Mu'aawiyah (Radiya Allahu Anhuma). Who killed Hassan (Radiya Allahu Anhu); is it because of this war that Hassan was killed?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

The doctrine of Ahlus-Sunnah wal-Jama'ah (the People of Prophetic tradition and the Community) is to refrain from arguing about what happened among the Companions of the Prophet to pray to Allaah The Almighty that He may be pleased with them, and to believe that they exerted efforts in adhering to the right. Whoever among them made Ijtihaad (sincerely exerted efforts in their judgments and rulings) and was right, he would get a double reward. And whoever made Ijtihaad and was wrong would be rewarded once.

 Thus, ordinary people are not supposed to engage in arguments on their affairs, especially given the bitter fact that many history books were charged with blatant fabrications and false reports about them, aiming at degrading those virtuous Companions . Such books pictured the disputes they were embroiled in as matters of individual infighting and mundane materialistic interests.

The Prophet said: “When my Companions are mentioned to you, then refrain (from arguing about them)." [At-Tabaraani] 

In light of this Hadeeth (narration), we think that what we have just said is enough and sufficient. As for the death of Al-Hasan ibn 'Ali ibn Abi Taalib he died a natural death and was not killed in that war or any other one. It was said that he died by poison without knowing the criminal, according to Al-Isaabah by Imaam Ibn Hajar .

Allaah Knows best.