Original sentence: The  historian Vegetius theorized, and has recently been supported by the historian Arthur Ferrill, that the Roman Empire – particularly the military – declined partially as a result of an influx of Germanic mercenaries into the ranks of the legions.This "Germanization" and the resultant cultural dilution or "barbarization", led to lethargy, complacency and loyalty to the Roman commanders, instead of the Roman government, among the legions and a surge in decadence amongst Roman citizenry
Edited sentence: The  historian Vegetius theorized, and has recently been supported by the historian Arthur Ferrill, that the Roman Empire – particularly the military – declined partially as a result of the ad-hoc use of Germanic and other mercenaries instead of trained and disciplined long-service soldiers.This "Germanization" and the resultant cultural dilution or "barbarization", led to lethargy, complacency and loyalty to the Roman commanders, instead of the Roman government, among the remaining troops and a surge in decadence amongst Roman citizenry