I think you mean the vigor of the Roman Empire began to decline when what happened. Although the sheer size of the Empire caused problems, the one event which marked its eventual decline was the decision to allow the Goths to cross over the Danube into the Empire in 376--one hundred years before the Empire finally came to an end. At that point, the Goths had been pushed against the Danube, the Eastern border of the Empire by the Huns. They requested and received permission to enter the Empire, but were treated by the Romans with great hostility and disdain. The Romans made no provision for their care or upkeep which led to a rebellion by the Goths. During the resulting Battle of Adrianople, the Goths defeated the Roman army, and the Eastern Emperor Valens was killed when the Goths set fire to a farm house in which he had been carried after being wounded. Incidentally, there is no evidence that they knew he was there or targeted him. At any rate, the loss of that Battle made the Goths a permanent fixture in the Empire. They did fight as mercenaries from time to time as did other Germanic Tribes, because (1) the Empire was too large to defend and (2) competing factions within the Empire often fought with each other and employed mercenaries to do so. So, if one had to pick a date and event, I would say it was the Gothic crossing of 376 A.D. An excellent resource is Christopher Kelly, The End of Empire: Attila the Hun and the Fall of Rome.