Complex sentence: The 1869 robbery marked the emergence of Jesse James as the most famous of the former guerrillas and the first time he was publicly labeled an "outlaw," as Missouri Governor Thomas T. Crittenden set a reward for his capture.
Split into simpler sentences: The 1869 robbery marked the emergence of Jesse James as the most famous of the former guerrillas turned outlaw. It marked the first time he was publicly labeled an "outlaw," as Missouri Governor Thomas T. Crittenden set a reward for his capture.