Why was Rome the largest empire in Europe by 117 A. D.

There were many factors that led to the rise of Rome and making it the largest empire including abled leadership, militaristic superiority, diplomatic relationship with neighbors, etc.

Explanation:

Once the Romans got victory over Carthage, there was hardly any nation left against which they had not to hold a clear advantage. The Hellenistic states dealt with small reserves of manpower because they were not unified with the native populations they have conquered and secondly they used a military system that could not stand against the Roman military structure. so there was hardly any competition left to Romans. Leaders like Augustus helped rebuild the city of Rome and protected its borders. The legal system was reformed to stabilize the empire.

the answer is a new goverment.