I agree that 5th century leaders such as Honorius were disgraceful, but there were also competent leaders such as Constantius and Majorian who tried to restore order, but ultimately failed.
Note the hedge in my statement: "early fifth century."
When Theodosius dies (AD 395), I feel that the Western Empire still had a great potential for lasting success and stability.
If Rome had a Stilicho (died 408) as emperor, instead of the worthless Honorius....
By the time of Aetius (died 454), however, Rome may have already been in a terminal death spiral and nothing could have saved her.
I will defer to Ian on this, however.
guy also known as gaius
Gee thanks Guy: no pressure then!
I think there may be some major differences between the periods under discussion.
As has been pointed out, in the third century a series of competent emperors - however short lived - managed to slowly piece the Empire back together again, both physically and mentally. After the 'Third Century Crisis' the majority of the Empire was happy enough - or scared of the emperor enough! - to continue to be 'productive citizens', paying taxes, supplying recruits for the army, and refraining from rebellion in any of its forms - although usurpations and cicil war did continue, especially during the 'reign' of Constantine and his sons. In addition, the barbarian invasions of the third century, although devastating, did not fundamentally damage the Empire's social and economic base as no permanent settlements were made, except on the Empire's terms. Furthermore, although the Empire had been riven by civil wars, the majority of the casualties were to be found in the army, which was often led in person by the emperor to face both internal and external enemies. Despite the constant usurpations, the Empire remained essentially united, with many of the differences between East and West being ignored. At the end of the century there was a feeling of renewal and confidence in the empire.
The fifth century is almost completely different. After the division of the empire between Valentinian and Valens the separation between East and West became an ever-growing division. This was not helped by the accession of two incompetent but relatively long-lived minors in 395 (Arcadius died in 408, Honorius in 423) and the succession of Theodosius II, who was not much of an improvement, in the East (408-450). The empire was not ruled 'jointly': instead, the two rival political courts generally dealt with each other in a hostile fashion - especially during the leadership of Stilicho in the West. Moreover, after Theodosius I's campaigns against the West, in effect the West was left to sort out its own problems without help from the East, again thanks largely to the policies of Stilicho in the West and his opponents in the East. Contrast this with events after the Battle of Adrianople in 378.
In addition, the barbarian 'invasions' of the West - especially the Goths in 401, and the Vandals, Alans and Sueves in 406 - resulted in large territories being lost for the purposes of taxation and army recruitment. When combined with the elite's withdrawal from military and political responsibility - especially in the paying of taxes and the supplying of recruits - the net result was the loss of the Western army and the growth of the 'Barbarian Kingdoms'. It is difficult to defeat an enemy in the long term when you are unwilling to take casualties in defeating him, relying instead on blockade and siege. Because they did not suffer from devastating casualties in war, the barbarian kingdoms were usually willing to re-open conflicts after only a short period of rest and renewal.
The growth of the barbarian kingdoms was helped by the desire of large segments of the population in Britain, Gaul and Spain to secede from a Roman rule which appeared to demand much in taxes but offered little in the way of protection from barbarian intruders, whether from outside or inside the Empire. Aetius was to spend a large part of his campaigning career putting down never-ending Bacaudic rebellions, a problem that was not on the same scale in the third century.
I hope this answers some of the questions!
Right, back to work - what was Valens doing in 370 .... ?