The Fall of the West by Adrian Goldsworthy

"...instead of inquiring how the Roman Empire was destroyed," asked Edward Gibbon in the Decline and Fall, "we should rather be surprised that it has subsisted for so long."  This recent but nicely old-fashioned history of the fall of Rome certainly bears this out: a litany of no-marks and nutters in charge; endemic civil wars and an institutionally terrible government.  Gibbon thought the sheer size of the Empire brought the whole thing down - Goldsworthy reckons the momentum was the only thing keeping it going.

It starts off looking at the 3rd century - a time of crisis, with frequent assassinations, different bands of troops proclaiming new emperors all over the shop, and constant fighting.  In the 4th century, things calm down a bit with a couple of strong emperors who know what they're doing - Diocletian sets up a tetrachy (four emperors!) which only works for as long he's the one in charge of them.  More civil war - then the rise of Constantine who turns the Empire Christian and, less famously, murders his wife by locking her in a sauna until she chokes to death.  The 5th century is when it all comes crashing down.  Attila the Hun, Vandals and assorted Goths.  The Empire carries on at Constantinople for a thousand more years after this.  But it's not the same - real Romans don't speak Greek!

Goldsworthy has a bit of a grumble about all the different theories historians have put forward about what went wrong - population decline, disease, movements of peoples outside the Empire.   All things which are very hard to discern in the historical record.  What they tend to ignore is what we what we do know about - interminable civil wars, which ran for most of these three centuries.  For Roman soldiers the big enemy was always other Roman soldiers. This was a colossal drain on resources, and would also have been devastating for Roman citizens living in the path of these warring armies.  And barbarians living on other side of the Danube or wherever couldn't fail to notice when all the legionnaires had abandoned the border to fight other legionnaires...

It's important to note that these emperors weren't fighting for a cause - Romans didn't really have any ideologies.  It wasn't even as if it was, say, a Christian emperor versus a pagan emperor.  They were fighting only for power and survival.  In previous centuries emperors had all come from the senatorial class.  To stop potential claimants, the senate was increasingly sidelined to make emperors more secure.  In fact, the opposite happened - lower class equestrians started grabbing the throne anyway and the pool of potential usurpers grew exponentially.  In the end any military officer who could sit straight on a horse had the chance to become a living god.  For a short time, anyway.

I've mentioned it in respect to this trash, but the original sin of the Roman Empire was succession.  Augustus never set up a good system for who becomes next emperor.  British history is obsessed with succession  - arguments about who's descended from whom, who's got a strong claim to the throne, who's the rightful heir.  It all might seem pretty ridiculous to us, but a very likely alternative would've been the anarchy and violence of the late Roman Empire.

Here's an interesting aside.  Like the popes, emperors grew sick of Rome.  Most of them in this period never even saw the Eternal City - they were either constantly on campaign (mostly against other Romans, of course) or holed up in Milan or, increasingly, Ravenna.  Poor old Rome - it became a shabby backwater in its own dying Empire.