Defences of the western Roman empire in 5th century

[Theodosius the Great]

Only in the late 5th c. we see that even that position is not tempting enough when Gundobad the Burgundian rather takes the job of king at home rather than first man in Rome. For me that's the real Fall of Rome embodied.

The Visigothic King Euric seems to have arrived at the same conclusion a full 10 years earlier before Gundobad had abandoned Italy. In the mid 460s Euric was openly annexing territory and allying himself with other foederate kingdoms against the Empire.

I'd love to know what sources Heather used to reach the total of 'only' 300 ships.

Apparently, he's referring to Western Emperor Majorian's failed expedition to reconquer North Africa. In 461, Majorian gathered 300 ships which were to launch from Spain but Gaiseric got wind of the plan and launched a preemptive strike.

I too am struggling with this idea. How long would it take for a message from Boniface to reach the Vandals and for them to decide to act?

Part of the problem is that Procopius is quick to point to treachery whenever disaster strikes. Procopius has to find some explanation for his audience. Another example where he cites supposed treachery is when he blames the Eastern commander Basiliscus for the defeat of the expedition sent against Gaiseric in 468. Basiliscus was just incompetent.

West and East never 'supported' the weaker half, they had by the second half of the 4th c. entered into a dangerous state of competition that would lead to the fall of the western half.

When Theodosius II died Valentian III wanted to rule the whole empire since he was the last male of the Theodosian dynasty. But Aetius opposed his efforts and the Eastern Empire didn't want him anyway. After the death of Valentinian III, Constantinople didn't know who to support in the West since there was a revolving door of imperial usurpers who couldn't gain the support of the Gallo-Hispano-Roman families.

~Theo