Article: The internal unrest reached its most serious state, however, in the two civil wars that were caused by the clash between generals Gaius Marius and Lucius Cornelius Sulla starting from 88 BC. In the Battle of the Colline Gate at the very door of the city of Rome, a Roman army under Sulla bested an army of the Marius supporters and entered the city. Sulla's actions marked a watershed in the willingness of Roman troops to wage war against one another that was to pave the way for the wars which ultimately overthrew the Republic, and caused the founding of the Roman Empire.

Now answer this question: What can be considered one of the causes that led to the downfall of the Roman Republic?
the willingness of Roman troops to wage war against one another