Attila, who had come to rule the Hunneric Empire in 434 A.D. through the death of his uncle, Rugila, crossed the Rhine into Roman territory. Up until then, Attila had wisely taken tributes from the Romans and their allies, the Visigoths. These tributes -- or payments for safety from raiding Huns -- had been negotiated with the Roman Empire to include at least 350 pounds of Roman gold. When the tribute was delivered, the Romans and the Huns would exchange prisoners and keep the peace for quite a while. However, Attila became increasingly unhappy with the Romans who gave refuge to his political enemies, and as the Roman Empire rapidly declined, the tributes became less and less substantial.
Attila, losing patience with the Romans, crossed the Danube in 440 in his first major raid. Since this coincided with the Vandal's sacking of the major Roman outpost of Carthage, it was duly timed, since it meant that Roman taxes flowing from both Carthage and Africa were lost during this period, further weakening the Roman state. As he pressed inward toward the heart of the Roman Empire, Atilla defeated the Roman army on three separate occasions and reached the outskirts of Constantinople. This activity withered the grip of the Roman Empire on its entire eastern front. Attila's raids included a ravaging of the major Roman strongholds of Thrace and Macedonia as well, further undermining Roman rule in the major stronghold of Greece.
When Attila began his attacks on Rome in earnest, the city of Rome had already seen wave after wave of invasions from the Visigoths, Vandals, Suebi, Alamanni and Burgundians. Their rule in both France and Spain had lessened to the point that it was in name only, mostly just for show, and with no real substantial authority. Attila's raids began thirty years after these initial attacks and were so brutal that the Roman populace took to calling him the "Scourge of God." Attila continued attacking Romans as he grew increasingly impatient with the tributes, which had escalated from 350 pounds to 2,000 pounds. When the crumbling empire couldn't meet the payments, Attila took it from them by force.
Living in the eastern side of Italy in a considerable palace, Attila made plans to strike at Constantinople -- now the seat of power for the Roman Empire. This was because provincial Roman rule had crumbled to such a degree that many previously conquered lands refused to send tax revenue to a Roman government that no longer worked. He died at his base near the Danube before he could enact his plan, drowning in his own blood from a fall during a drunken stupor. His sons subsequently divided the kingdom but could not hold the Hun's empire together.