Complex sentence: This attitude partly changed only after he had to flee from Mecca to Medina.This "hijra", or migration, would be a turning point in the Prophet's mission and would mark the very beginning of the Muslim calendar.
Split into simpler sentences: This attitude partly changed only after he had to flee from Mecca to Medina. This hijra would be a turning point in the Prophet's mission and would mark the very beginning of the Muslim calendar.