Either or.
I thought Rome fell because of over expansion and over reliance on Gothic barbarians for protecting the borders? Is there a difference between the fragmentation or the fall?
There's a lot of reasons why Rome fell but that's not really why. Rome fell, to put it simply, because of political instability mixed with a lot of bad luck, mixed with increased pressure from more organized barbarian entitities pressing on the frontier and forced to find shelter across the border due to climatic changes and to escape Hunnic expansionism. Rome's army was perhaps better quality wise in the 4th and 5th century than in previous eras. They had no more reliance on barbarians than they had on non-Roman auxillaries in previous centuries. Reliance on, say, the Franks to defend the Rhine frontier paid off quite well: the Franks defended the frontier mostly very well. Even in the crossings of 406-407, Frankish forces came very close to defeating the forces attempting to cross, only failing when reinforcements arrived to turn the tide against them. Now onto why the west fell...
Basically, the west was hit with a quick succession of body blows, rather than one or two knockout punches. Towards the end of the 4th century, western forces lost 2 civil wars in quick succession to Theodosius, hurting their manpower. Stilicho, being half Vandal, would have trouble with the Roman senate basically out to get him, and so he faced trouble with getting money and men to bulk up his numbers. Then you have the Rhine crossings in December 406, and Stilicho's inability to deal with it due to Constantine III crossing into Gaul and then basically letting them into Spain where they ran rampant. Political instability that would result in the next 2 decades aside (fall of Stilicho, sack of Rome,rise of Constantius, his unexpected death, brief civil war and rise of Aetius), Alans and Vandals getting control of Spain (and the Goths moving into Aquitaine) seriously hurt Rome's revenue and further limited their manpower pool. Losing North Africa however was a death blow, since that was the breadbasket and wealthiest province in the empire. With North Africa, the empire could theoretically survive indefinitely. Without North Africa, they were on borrowed time (468 being the last best chance for the west to survive).
But back to the third century, I'll take a shot at it:
-260 CE: Postumus proclaimed emperor. Marches on Italy, defeats and kills Gallienus.
Odenathus, who initially supported Gallienus, gives his public support to Postumus, but Postumus becomes wary of the man who supported his rival
-261: Rhine frontier even weaker now from civil war, facing increased pressure. Postumus moves to try and stabilize Danube frontier first.
262: Postumus killed in battle against the Goths.
-The Praetorian Praefect, Victorinus[1] is proclaimed emperor when the soldiers regroup. A temprorary treaty is patched up with the Goths as he tries to solidify domestic control.
-A usurper arises in the east. Odenathus dutifully crushes the revolt, and is awarded with the title of
totius Orientis imperator[2]
264: With a weakened Rhine frontier, Franks resume their raids. [3]Victorinus marches on to that frontier in an attempt to stabalize it.
267: Massive invasion of the Balkans begins by the Goths and Heruli begins. It starts with an Heruli expedition from the black sea ravaging much of Greece. The invaders were defeated at sea, but on land the Romans struggled. They defeated the Heruli and Nessus leading to them coming to terms, but the struggle had only begun.
268: A second Gothic invasion begins, achieving much greater success. After raiding Byzantium, they break into the Aegean and ravage as far south as Rhodes and Crete. Before the emperor can respond, Thessaloniki and Cassandreia are taken[4] and sacked.
-Discontent with Victorinus' failures mount, and his own men murder him on the march to take on the Goths and Heruli. An obscure officer by the name of Claudius (OTL Claudius Gothicus) is proclaimed emperor.
-268: Alemannic and Juthungi raid into Italy defeated by hastily raised force by Aureolos, former officer under Gallienus who had switched sides to Postumus early on. Aureolus is proclaimed emperor by his troops.
269: Claudius fights an indecisive battle with the Goths at Salonica. Dies shortly afterward of the Cyprian Plague. His brother Quintillus is proclaimed emperor with the backing of the officer Aurelian. Quintillus is soon ambushed however by Gothic forces, and suffers a crushing defeat, with Aurelian dying and himself committing suicide after the battle (other narratives say he was assassinated by his own officers).
-The Goths, and Heruli pour into Greece.
270: After a brief sojourn west while Gothicus and Quintillus were tied down in the Balkans, Aureolos heads into the Balkans.
-While fighting Vandals in Pannonia, he learns of an Alemannic and Juthungi invasion of the Po Valley, and quickly heads back into Italy, being ambushed and defeated at Placentia.
-Rome is sacked by Juthungi and Alemannic forces. Aureolos is murdered by his own troops, and an officer named Saturninus[5] is proclaimed emperor. He shortly thereafter chases down and defeats the Juthungi and Alemanni, forcing them back outside of Italy.
271: Roman forces are at this point completely absent from the Balkans. Saturninus' attempts to re-establish control there meet with limited success.
-increased pressure on the Rhine frontier causes MArcianus to be proclaimed emperor. His reign is shortened however by a defeat to the Franks and subsequent murder by his troops, causing a breakthrough on the Rhine frontier.
-governor of Britain, another Claudius, proclaims himself emperor but does not immediately sail to Gaul.
272: Saturninus falls victim to the plague
-Constantius declared emperor in North Africa, sails to Italy and seizes power
273: Franks and other barbarian allies break into Spain once more, pouring into the province a la the barbarians of 406-407.
274: Zenobia seizes Byzantium, ostensibly in support of Constantius against the Goths, but in reality to obtain control of the Bosporous and assist her in repelling Gothic raids on northern Asia Minor.
-Constantius focuses on regaining control of Spain. Achieves some success before arrival of Claudius in Gaul forces him to cut a deal with the Franks as he turned north.
-Salian and Ripaurian Franks partition controlled parts of Hispania between them
276: Hostilities reopen in Spain. Ripaurian Franks begin raiding Balaeric Isles and Numidian coast. Salian Franks dealt decisive defeat and merge with Ripaurian Franks. Constantius settles once more as he turns his attention to Gothic raid into Italy.
278: Franks cross into North Africa. By 280, most of North Africa under their control. The rest falls to them in 284.
288: Gruthungi Goths cross into Italy and defeat Constantius' successor, Quietus. Defeated in turn by Juthungi-Alemannic invasion which takes over Italy.
The Roman empire, aside from holdouts in Spain, Gaul, and Britain, ceases to exist. The Palmyrene Empire led by Zenobia remains a powerful force in the east.
Not sure how plausible all this is, and I'm sure LSCatalina will come on here and assess that, but this is what I could come up with.
[1] He was appointed Praetorian prefect under Postumus's Gallic Empire, so Postumus must have had a high opinion of him-it would make sense for him to appoint an ally like Victorinus to this position in the empire here as well.
[2] similar to OTL
[3] OTL the Franks stopped raiding for a decade after Postumus defeated them in 260 and due to the Gallic empire's stabilizing of the borders. Here, the forces withdrawn by Postumus' march on Italy aren't fully replaced, leading the Franks to try raiding again earlier.
[4] OTL they were close to being taken before news of Claudius Gothicus' arrival prompted them to retreat.
[5] Made up guy.