Many barbarian warbands crossed the Rhine-Danube border into the Roman Empire, causing widespread chaos. Attempts to defend against or integrate these groups failed. The 5th-century invasions were a catastrophic event.
(1 of 2) Next argument >
During the late 4th century the Goths arrived on Roman border, looking for a new home. They had been pushed West by the marauding Huns. The Goths never got a reply from the Roman Emperor Valens, so invaded anyway. They would go on to deliver a catastrophic defeat to the Romans at the Battle of Adrianople. Although some Goths fought with the Romans during subsequent invasions, they were never controllable and the Ostrogoths set up an independent kingdom in Italy, after the Visigoths sacked Rome in 410.  The instability caused by the Goths was compounded by further invasions by the Suevi, Alans, Angles, Bugundians, Lombards, Franks, Maygyars, Saxons, Vandals — and finally the Huns among others — over the next century. These invasions took place at different times in different regions, and an increasingly fractured politically unstable Roman state could not do anything to stop them.  Rome was sacked a second time in 455 by the Vandals, weakening Rome further. The Huns were brought to a standstill in 451, but only with the help of multiple barbarian groups, many of whom formed their own kingdoms across the empire. Multiple invasions during the 4th and 5th century became impossible to manage. The Goths in particular undermined the Roman military internally weakening the empire.
The scale of the problem facing Rome in Late Antiquity has been exaggerated. The threats facing Rome during the later Roman Empire were no greater than the early military threats from Germany and Persia. Goth troop numbers were no greater than the empire had faced before. Rome’s inability to handle the invasions was the main difference. A slow decline rather than one major event caused the fall of Rome. Blaming Barbarians alone is an over-simplification.  The Roman empire fell for many reasons. A multi-causal decline is the only thing that can explain the barbarians' easy victories over the Romans.
[P1] In the late 4th and early 5th centuries many barbarian tribes showed up on the border simultaneously [P2] The Roman Empire was overwhelmed [C] The arrival of many tribes at once was a cataclysmic event