under what circumstances is it right to fight against other muslims. (for example the cival war between the taliban and the northers alliance)

The Northern Alliance were Baagis (rebels). After joining the coalition of
the Kuffar, they have lost their Imaan and have fallen in the ranks of the
Kuffar. Fighting them is no more different from fighting the rest of the
Kuffar.

and Allah Ta'ala Knows Best

Mufti Ebrahim Desai
FATWA DEPT.