Late Roman Army during the 5th century

[Flavivs Aetivs]

The Army had to increase it's size by approximately 1/3rd when the Sassanid Empire came about in (238?).

The Sassanids came to power in 226 CE and had already fought a war against Rome prior to 238. I don't think there was any increase in the army's size until the time of Diocletian, and it reflected heightened threats in Europe as well, not specifically the Sassanians, who after their midcentury rampage pretty much quited down for years.

That's what caused the collapse of the 3rd century.

It was caused by a number of factors. I don't think the problem around midcentury was a shortage of troops; it was more the ineptitude of emperors like Decius and Valerian.

The loss of africa meant the loss of revenue - Africa produced about 2-3 million Solidi prior to the loss. (3 million solidi would have supported an army of 90000 based on a rate of 30 solidi per annum).

Yep--funny they didn't seem to have anywhere near that many in 410, or when Aetius had to turn to outsiders, the Huns, in the 430s. Not enough citizens seemed to care anymore.

The Empire was faring just fine until they lost africa - prior to the loss of Africa there was still a chance to recover the Empire.

The western Empire was already fatally afflicted by about 408. It just no longer seemed capable of summoning the strength, especially indigenous strength, to maintain its position for long. The loss of Africa marked the point at which the inevitable erosion of the WRE reached a fatal point. It can be likened to an infection reaching a vital organ of some patient too weak to resist.

Aetius did not have to rely on hunnic mercenaries - he merely had an edge with them. The Roman Army in the 430s (this is speculative and theoretical though) mainly consisted of The Old Gallic Field Army and many of them were paid as a private retainer cause the government couldn't afford it.

The Barbarians were not a problem - only chieftans had armor and the raids accross the borders were still mostly petty until the Goths came down the black sea. The army's increase was in the East, read Peter Heather's Fall of the ROman Empire and his EMpires and Barbarians, he's the one who proposed the increase in the size of the army to counter the sassanids who were a power ont he same scale as Rome. The Parthians were never a strong nation and Rome rarely had to deal with them on such a large scale. The army was the focus of Rome's economy and the required increse to counter the centralized army of the Sassanids meant inflation, and other factors which led to Economic Collapse. The Rapid transition of emperors somewhat played into it, yes, but many of them were strong Military leaders.