The single most important historical event in The Bastard's Blade is the fall of the Roman Empire. In 362 AD, Rome encompassed the entire Mediterranean and half of Europe. Little more than a century later, those borders had shrunk by almost two-thirds.
In his Atlas of Medieval History, Colin McEvedy makes a crucial point: "What has to be explained is not that the Empire fell apart, but why the bits never came together again." (McEvedy 9) That explanation has little to do with administration, politics, food supplies, Christianity, corruption, or moral decay that is so often associated with Rome's decline. Instead, it boils down to military losses and the untenable situation those losses created.
In the 4th and 5th centuries, the area shown on the Shepherd Map is like a chess board with too many pieces. You have Rome occupying about half of it, while the rest is a cluster of disparate Germanic tribes and Huns, all vying for living space. No matter who gained what territory outside Rome, it was Rome who had to deal with the victors, because its borders touched everyone.
At the end of the 4th century, the Goths (strongest of the German tribes), unable to press south and west across Roman borders, struck out east, where they clashed with the Huns. Other Germanic tribes, like the Angles and the Saxons, contented themselves with raiding Britain and Gaul. Yet the Romans held their borders, having realised that further expansion into Germania was an unproductive effort. 
At the beginning of the 5th century, the Huns moved westward in force, dislodging the Goths, who found themselves pressed against Roman borders, as well as those of other Germanic tribes. This was the beginning of the end for Rome, whose infantry legions were outclassed by the Goth and Hun cavalry. Having no cavalry of their own, Rome began hiring Germans and Huns to fight their battles for them, essentially paying one enemy to fight another. It was a delicate balancing act of warfare, negotiations, and bribery.
Unfortunately, it was never-ending, owing to a steady supply of roving bands of opportunistic barbarians all too ready to extort Rome for cash in exchange for good behaviour. And, of course, no matter which side won—Roman-sponsored mercenaries or external aggressors—the winners were still barbarians, and they still needed a place to live. Not surprisingly, it wasn't long before Rome found itself in control of the military situation only as far as she could pay to influence it. And the Imperial coffers were running dangerously low.
By the middle of the century, the Huns (under Attila) occupied an area stretching from the Russian steppe to the Baltic. The resulting dislocation of the Germans, and the inability of Rome to pay for its abatement, had started to tell—much of Spain, a third of Gaul, all of Britain, and a bubble of North Africa surrounding Carthage  was lost to ravaging Germans looking for a home.
Attila's death in 453 relieved some of the pressure on the Germans, but for Rome, it was too late. By 476 AD, the Western Empire was under the de facto control of Germanic warlords, who had expanded their beachheads along the outskirts of the Roman Empire to encompass vast territories. One German general, Odoacer, seized all of Italy simply by ceding token submission to the Eastern Emperor at Constantinople. At the same time, Spain and Gaul were under the control of the Visigoths; Britain was the target of raids by Angles, Saxons, Jutes, and Frisians; native British chieftains had fled the isle to carve out the state of Brittany in Gaul; the Vandals migrated to North Africa to effectively control trade across the Mediterranean; and a host of Germanic tribes were slowly consolidating the Frankish kingdom in continental Europe.
All the while, the Eastern Roman Empire remained highly solvent. It's not that they weren't beset upon by the same external marauders that had ravaged the West. Indeed, they had their share of headaches from the Huns, the Ostrogoths, the Alans, the Persians, and the Arabs. What kept the East intact was that it was rich: money pouring in via the Silk Road and Indian spice trade allowed it to pay its aggressors premium fees to be spared the same fate as its Western counterpart.
But there was also one other, unanticipated, card the East chose to play: By selling out the West, it could encourage the barbarians to ravage elsewhere. Whenever the Ostrogoths got ornery, the Eastern Emperor could simply hire them to raid the Germanic kingdoms, effectively killing two birds with one stone: rid the local borders of marauding bands and rid the West of barbarian warlords. Regardless of the outcome, the Eastern Emperor had one less band of troublesome heathens to deal with. 
Thus is the state of affairs in 486 AD. I apologise for presenting a mound of text, but I believe this summary will help me be more succinct in, and make more sense of, the area descriptions to come.
Next up, how Barael's Europe differs from the real world.
Listening to: Robert Walter's 20th Congress, Money Shot
- An admonishment as old as the first Roman Emperor Augustus in 14 AD. Augustus figured that the Empire was pretty much just the right size, and while its armies could expand it, expeditions beyond the borders revealed that there wasn't much out there worth fighting for. The aborted conquest of Germania is a good example: after losing 3 legions to the effort with nothing appreciable to show for it, Augustus was satisfied to fortify the frontier along the Rhine and call it a day.
- This wasn't just the loss of territory, it was the loss of Rome's major wheat supply. By this time, Rome was already "living from hand to mouth" (McEvedy 12) and so while the Vandals in North Africa were not an immediate military threat, the Romans nonetheless found themselves forced to pay tribute to ensure stocked granaries.
- In fact, this is precisely what happened in 488, when Eastern Emperor Zeno paid the Ostrogoth King Theodoric the Great to invade the Kingdom of Odoacer. Theodoric did so with zeal, and by 493 was the ruler of a new kingdom consisting of his old Ostrogoth dominion, Italy, Sicily, Dalmatia, and Provence. It's worth noting that Theodoric supposedly killed Odoacer with his own hands at a banquet set to mark peaceful relations between the two. Seems Odoacer didn't see that one coming, despite having been besieged by Theodoric for just shy of 5 years straight.
McEvedy, Colin. The New Penguin Atlas of Medieval History. Penguin, 1992.