Young Huns go for it
The great thing about explaining the fall of the Roman Empire is that it inspires almost everyone to have a crack. Peter Heather doesn't actually mention the theory that it was all down to the Romans becoming mentally infirm after drinking from lead water pipes, but it's safe to say that doesn't pass the two questions any theory needs to explain. Why did the Empire fall in fifth century, rather than earlier or later? And why was it only the Western Empire which fell? (The Eastern half continued on, eventually becoming the Byzantine Empire, which Heather feels to be a successor state, and who am I to argue?)
These two questions knock out many favourite theories. Roman licentiousness can't be to blame, as the first century emperors excelled at this - performing a selection of their favourite violin solos while the capital got torched, promoting horses to consul and poisoning each other's figs. The later emperors, by contrast, were a much less lively bunch.
The second knocks out the I blame the Christians argument of Edward Gibbon. Which is sad, because pointing out that the advent of Christianity as a state religion coincided with the Empire's decline had good irritation value. Saint Augustine wrote City of God while the Vandal army was camped out in the suburbs of Hippo. This book started a backslide of Christian thought: having been more than happy to embrace the Empire, they were now starting to put some theological distance between Church and State. But the Eastern Empire was more religious and Christian than the West, and it survived.
Peter Heather refreshingly takes the direct approach, which is that the Western Empire fell because it was overcome by a seemingly endless stream of mainly Germanic invaders. As provinces fell, the Imperial treasury became empty, its ability to pay its armies diminished, and the crisis intensified.
Heather identifies the Huns, steppe-dwelling horsemen, as the catalyst behind all the main crises in the century long fall. A build up in the population of German tribes had increased their danger to the empire. The arrival of the Huns on the Volga led to the collapse of various (German) Gothic kingdoms in the Ukraine. Survivors fled south over the Danube. Conflict with the Empire forced amalgamation of these tribes, and the defeat by these Visigoths of the Romans at Adrianople was a turning point. With Valens, the Emperor, dead, it's difficult not to see Adrianople as the beginning of the end.
But the end would be a long time coming. The Visigothic War was ended, only for a worse Germanic crisis to erupt in the decade after 400. Another Visigothic war in 401-402 was worsened after group of Vandals, Huns and Alans walked over the frozen Rhine on the portentious 31 December 406. In 410 the Visigoths, led by Alaric, sacked Rome, though it was apparently a comparatively civilised affair. Peter Heather sees the arrival of the Huns in Hungary as having sparked off this crisis, but he doesn't appear to have any evidence that they were there before 425.
We then move on to Aetius, the "last of the Romans". He is traditionally seen as an untouchable hero after his defeat of Attila's Huns in 451, but as someone (can't remember who, sorry) once said, he was only the last of the Romans because he didn't leave any Romans to come after him. In particular, Heather lets Aetius off lightly for signing a ridiculous peace treaty which let Geiseric's Vandals stay in Morocco. They were then able to wait till Aetius went off cruising for Germans in Gaul and then marched into Carthage, taking the Romans' most financially important provinces. For all Aetius' heroism against Attila and his fruitless plans to retake North Africa, it was this decision which did for the Empire. Without the excess finances to buy large armies, the Western Empire was pretty much sunk.
The disappearance of the Huns after Attila's Huns as marking another stage in Rome's decline. Aetius had used the Huns to keep the Germanic tribes in check. With the formidable Roman army much diminished, modern Britain, France, Spain and north Africa had been lost. This left only the Eastern Empire which could recover the situation in the West, but once Anthemius' fleet had been lost off Carthage, the East was bankrupt and the West was finished. A few years later in 476, the last Emperor, Romulus Augustulus, was pushed into retirement and the Western Empire was over.
Another factor in the fall was the amalgamation of the Germany tribes into ever-larger groups. Visigoths, Ostrogoths, Vandals and Franks were recent inventions, as groups of invaders realised they would rather hang together than hang separately.
Peter Heather has a fun and informal (well, for a historian) style. Traditional historians probably wouldn't have approved of Heather's amusement at the Roman habit of inviting barbarians to banquets and then sticking the fadge on them, but you've got to admire someone who can come up with a heading like "Thrace: the final frontier". Definitely worth reading.