Did rival Muslim rulers have any religious premise to justify the wars between their umma?

Much blood has been shed between Muslims in rivalry over who should lead the umma. Before fighting over politics, the premise of the requirement of a single ruler for all of the Muslims was never established. Expecting any ruler to walk in the prophet’s shoes is impossible. For example, what was the premise for Ali ibn Abu Talib not allowing Muawiya ibn Abu Sufyan to run his own caliphate in Syria and causing the battle of Siffin? Was this premise worth 1 drop of Muslim blood? Likewise, after the death of Ali ibn Abu Talib, what was the premise for Muawiye not allowing Hasan ibn Ali to have his own caliphate in Kufa, or Medina? There seems to have been an inclination to use religious leadership to expand the scope of political leadership to the entire umma. The premise seems to be the prevention of religious disorder termed ‘fitna’ where slaughter is preferred to prevent religious disorder (Quran 2:191). However this wahi was revealed in Mecca against the polytheists who were preventing the Muslims from visiting the Kaaba for Haj. Using this aya to spill the blood of fellow Muslims who did not differ in religion, but only political allegiance to a political leader seems very dubious. Did rival Muslim rulers have any religious premise to justify the wars between their umma?