[KeettyGlots]

what is the punishment if a muslim wages war on another muslim without a just cause?

No Muslim would wage a war on another Muslim without a just cause, if a "Muslim" waged a war of aggression against other Muslims without any cause then he is not a Muslim but a hypocrite and if he kills them then his punishment in this life is death according to Shari'ah.
The life of a Muslim is more important to Allah than the destruction of the Ka'abah.