The huge quantities of water needed for this project had to be contained in large reservoirs, and the standing water became stagnant. Stagnant water is the ideal environment for breeding mosquitoes, the carriers of malaria. Malaria began manifesting itself at epidemic levels, weakening and killing a large percentage of the population. .
When one thinks of the Roman Empire, the first thing, which may come to mind, is the strength and stability provided by its army. It is widely held that the fortunes of Rome were directly tied to the prowess of her military. Many scholars have maintained that in the later period of the Empire, Rome's military might have steadily declined for a variety of reasons. In the end, it was this lack of security, which allowed the barbarian hordes to topple what had once been the mightiest empire in the world.
Historian Arthur Ferrill has stated this theory succinctly. "The destruction of Roman military power in the fifth century A.D. was the obvious cause of the collapse of Roman government in the West". He contends that a massive the Roman army, with perhaps 200,000 men, eventually disentegated into an unorganized rable. The most obvious factor in the falling apart of the army is the atrition sustained in the numerous conflicts with the Barbarians. Added factors such as the reliance on mercenaries, the fragmentation of the soldiers' loyalty to numerous power centers, and lack of incentive for Roman Citizens to enlist, combined to destroy the Army as a viable unit. .
One of the most important factors in the decline of the military was the lack of recruits from "Romanized" backgrounds. One problem was the granting of citizenship to all free men within the empire. Originally, non-citizens who served in the Roman army would be granted citizenship at the end of their enlistment. "It was this desire of the provincials to acquire Roman citizenship that accounts for the fact that during the first two centuries voluntary enlistments sufficed to keep the armed services up to establishment.