Final answer:
The breakdown of the Roman military involved soldiers shifting their loyalty from the Senate to individual generals who promised rewards, leading to political involvement and power struggles that contributed to the fall of the Republic and the rise of the Roman Empire.
Step-by-step explanation:
As the Republic grew more unstable, the breakdown of the military manifested primarily in the form of its involvement in politics and generals vying for power. This period saw a shift from soldiers being loyal to the Senate and the ideals of the Republic, to becoming career soldiers with loyalty to individual generals who promised them tangible rewards. These generals, such as those during the time of the First Triumvirate, used their armies to project their influence and vie for control over the state, which continued to exacerbate internal conflict and undermine the republican system of government. The ultimate culmination was seen in the emergence of the Roman Empire under the strong hand of one remaining leader after a series of civil wars and political unrest.
The key cause of the military's breakdown stems from decades of territorial expansion, where the military system of republican Rome declined after the Punic Wars due to the decreasing number of economically independent Roman citizens capable of serving as soldiers, leading to a rise in career soldiers motivated by rewards from their commanding generals.
This disintegration of the traditional military ethos and structure facilitated the eventual fall of the Roman Republic as the power dynamics shifted away from collective governance towards personalized authority figures, setting the stage for the establishment of the Roman Empire.