The late Roman Republic was plagued with civil wars for approximately a century. What made these bloody conflicts possible?
The Roman civil wars were very different than the civil war in the United States. The Roman internal conflicts were not about political issues or moral principles. The Roman civil wars were about personalities.
The typical Roman soldier had a primary loyalty, not to the Roman Republic, but to his general and officers. If two generals decided to compete for power, a civil war arose. This pattern culminated in the Battle of Actium.