Final answer:
The most significant difference between the legions of the Roman Republic and the Empire was the transition from a volunteer force to a professional army, with the Imperial army receiving regular pay and retirement benefits, and having a defined term of service. The professionalized Imperial legions included a significant number of auxiliaries alongside Roman citizens, promoting a broader Roman identity.
Step-by-step explanation:
Differences Between the Republican and Imperial Legions
The most significant difference between the legions of the Roman Republic and those of the Empire was the transition from a volunteer force to a standing, professional army. During the Republic, citizen-soldiers were mustered only during times of conflict and were motivated by patriotism and loyalty to the Republic. The expansion of Rome and social changes led to the decline of this system, as wealth and land were increasingly concentrated in the hands of elites, exacerbating debt and poverty among the populace.
Following Augustus's reforms, the legions became a professionalized force with regular pay, retirement benefits, and a fixed term of service, which was 20 years followed by 5 years in reserve. The soldiers of the Empire, known as legionaries, had a career-long loyalty to their generals and the promise of tangible rewards, including a significant retirement bonus and land. This professionalization aimed to secure loyalty and a stable military structure, capable of defending and expanding the sprawling Empire.
In contrast to the Republican legions, which drew primarily on Roman citizens, the Imperial legions were aided significantly by auxiliaries: non-citizen subjects who were integrated into the military, further promoting a shared Roman identity. The commanding structure also evolved, with legionary legates, often political appointees, heading the legions, while the centurions, particularly the First Spear, held significant influence over tactics during battle.