Mu'awiya's opposition to the Imam made him a baghi

Mu''awiya''s opposition was clearly religious because it was a direct challenge the Head of the State. This can be proven from the hadith of Rasulullah (s):

"O Ali! Soon a rebellious group will fight against you, you will be on the truth. Whoever does not support you on that day will not be from us" [Kanz al Ummal, by Ali Muttaqi al Hind quoting Ibn Asakir, hadith number 32970]

The very fact that Mu''awiya rebelled against the Ul IL Umr Ali (as) (you deem to be the 4th Rightly Guided Khalifa) clearly proves that it therefore was a religious dimension, hence Rasulullah (s)''s referral to this group as "Baghi".

Mu''awiya''s open rebellion to the Khalifa in violation to the Qur''an clearly proves this was a religious matter, and Imam Ali (as) was entitled as Ul il Umar to quash his insurgency. Imam Ali (as)''s actions were religious and in accordance with the dictates of the Holy Qur''an (Yusuf Ali''s translation):

"[al-Hujurat 49:9] If two parties among the believers fall into a fight, make ye peace between them: but if one of them transgresses beyond bounds against the other, then fight ye (all) against the one that transgresses until it complies with the command of Allah; But if it complies, then make peace between them with justice, and be fair: For Allah loves those who are fair (and just)".

This verse is absolutely clear that it is a religious duty to fight baghis (rebels), of which Mu''awiya was the Head. So Imam Ali (as)''s decision to fight Mu''awiya was RELIGIOUS and was supported by the Qur''an.

Ahlisunnah.org/ahlibayt states:

"Therefore, if Hazrat Ali bin Abu Tleb (Radhi Allahu Anh) himself does not see the conflict religious nor his political opponents as Kafirs, then the love which Shi''ah claim to have for him and the claim that they follow him, is an unproven Chapter from their own sources. For if they do indeed love Hazrat Ali (Radhi Allahu Anh) they will hold his views in this matter too, but they are people of no understanding".

Adapted from the book: "Historic Background of Shiism"