In his speech On Imperial Rule (or On Monarchy), Synesius of Cyrene offers some advise to the emperor Arcadius (r.395-408). More information about this speech, together with a brief summary, can be found here.
Throughout this speech, the word "Scythians" refers to the Tervingian Germans (who would later be known as Visigoths), whereas "king" refers to emperor.
The text is offered here in the translation by A. Fitzgerald. The four-digit numbers are page numbers of the Migne edition.
On Imperial Rule 15
  But first let all be excluded from magistracies and kept away from the privileges of the council who are ashamed of all that has been sacred to the Romans from olden times, and has been so esteemed. Of a truth both Themis, herself sacred to the Senate, and the god of our battle-line must, I think, cover their faces when the man with the leathern jerkin marches in command of those that wear the general's cloak, and whenever such an one divests himself of the sheepskin in which he was clad to assume the toga, and enters the council-chamber to deliberate on matters of State with the Roman magistrates, having a prominent seat perhaps next to the consul, while the lawful men sit behind him. Then again such as these, when they have gone a little way from the assembly, are again attired in their sheepskins, and once in company of their followers, laugh the toga to scorn, and aver that they cannot even draw the sword in comfort with it.
 For my part I wonder at many other things, but not least at this our absurd conduct. All this is in the face that every house, however humble, has a Scythian for slave. The butler, the cook, the water-carrier, all are Scythians, and as to retinue, the slaves who bend under the burden of the low couches on their shoulders that their masters may recline in the streets, these are all Scythians also; for it has been proved of old that theirs is the most useful race, and the fittest to serve the Romans. But that these fair-haired men who arrange their locks like the Euboeans should be slaves in private to the same men whom they govern in public, this is strange, perhaps the most incredible of the spectacle, and I know not what sort of a thing the so-called riddle may be, if this is not one.
 In Gaul Crixus and Spartacus practiced the calling of arms in dishonor, in order to become the victims of the Roman populace in the arena, but, when they escaped and bore a grudge against the laws,  they engaged in the so-called Servile War which became the most calamitous campaign of those which the Romans of that time encountered. Against these slaves they had every need of consuls, of praetors, and of the happy fortune of Pompey, for their city was nearly ravaged off the face of the earth. And yet those who revolted along with Spartacus and Crixus were not of the same race as they, nor of the same race as each other. Notwithstanding this, the fate which they shared in common furnished them with a pretext and made them of the same mind.
 It is, I suppose, in the nature of things that every slave is the enemy of his master once he has hopes of overcoming him. Is the case then the same with us also? Are we nourishing on an altogether greater scale the germs of untoward troubles? Remember that in our case there are not merely two men, and those dishonored individuals heading a rebellion, but great and pernicious armies who, kinsmen of our own slaves, have by evil destiny poured into the Roman Empire,note[A reference to the battle of Adrianople.] and furnished generals of great repute both amongst themselves and amongst us, 'by our own coward nature'.note[Unidentified quotation.]
 Consider also that in addition to what forces they already possess, they may, whenever they will, have the slaves as soldiers, right reckless and courageous ones too, who will perform the unholiest deeds to glut themselves with independence. This fortress of theirs you must pull down; you must remove the foreign cause of the disease before the festering abscess actually declares itself, before the ill-will of these dwellers in our country is exposed. For evils may be overcome in their infancy, but when they progress they gain the upper hand. The army must be purified by the emperor, as is a heap of wheat from which we separate the coarse grain and what other elements sprout side by side with it, ruinous to the noble and legitimate seed.
 Now if I seem to you to give counsel that is no longer easy to follow, have you not considered to what sort of men and to what sort of race those belong of whom I am speaking to you who are their king? It is a race whom the Romans conquered, and from such conquest their name has been renowned amongst men: further, they govern all men with whom they come in contact, both by their strong arm and by their will, and they have occupied the earth as do the gods, according to Homer,
Encountering alike the insolence and goodwill of men,note[Homer, Odyssey 17.487.]
 As to these Scythians, Herodotus says that they are all tainted with a feminine malady,note[Herodotus, Histories 1.105; Synesius makes a mistake, because the historian only refers to those Scythians who captured Ascalon in Palestine.] and we ourselves see this. These are the men from whose ranks slaves were are recruited everywhere, and who have never owned any land. Hence the proverb "the Scythian wilderness", for the are always fleeing their own country.
 The old historian tells us that the Cimmerians first drove them from their settlements, then others in turn did the same, then the women,note[Synesius refers to Herodotus, Histories 1.15 and 4.10.] then our ancestors, and lastly the Macedonian.note[Reference to the Greek colonization of the northern shore of the Black Sea and Alexander's campaign in Sogdia.] Forced by some of these they advanced towards inland nations,  and by others towards those without, nor do they stop until they are delivered up by their pursuers in turn to other nations right in front of them. When indeed they fall suddenly upon those who are not expecting them, they strike them with confusion for some time, as they terrorized the Assyrians of old, and the Medes, and the Palestinians. But in these days they have come to us, not to challenge us in battle, but as suppliants, for they have again been driven away from their country.note[Two tribes were living in the formerly Scythian country: the Tervingians (later called Visigoths) and the Greutingians (or Ostrogoths). The latter became part of the federation of the Huns; the former, pushed forward by the Huns, had requested for safety in the Roman Empire.]
 Now that they have found, I do not say the arms, but the dispositions of the Romans softer, as was perhaps to be expected when dealing with suppliants, this untutored race has made the natural repayment. It has become insolent and has ignored our kindness. When, then, they paid the penalty for this conduct to your father,note[Theodosius I defeated the Visigoths in 379, and gave them land in 382.] who took up arms against them, they again became objects of compassion, and sat at his feet in suppliant guise together with their women, and he who had been their conqueror in war, was quite overcome by pity. He raised them up from their prostate position, made allies of them, and accounted them worthy of citizenship. Moreover, he threw open public offices to them, and made over some part of the Roman territory to their bloodstained hands, expending the magnanimity and nobility of his nature upon a work of clemency.
 Unfortunately the barbarian does not understand chivalrous conduct. From the very beginning till now these men have treated us with derision, knowing both what they deserved at our hands, and what they were assumed to deserve; and this reputation of ours has encouraged their neighbors to make their way hither. Now hordes of foreign mounted archers keep pouring forth seeking out our easy-going people, begging for their indulgence and pointing out the case of these scoundrels as a precedent for it.
 The evil seems to be growing to the point of 'persuasion by force', as the popular phrase has it. No matter for the phrase; for philosophy must not quarrel over terminology when she is seeking something so serve her thought, even if she has to help herself by picking a phrase out of the gutter, so long as it is perfectly clear and appropriate to the matter in hand.
 Well then, how shall we find it other than a difficult task to recover military prestige, and 'to drive from hence these ill-omened dogs'?note[Homer, Iliad 8.527.] Only hear me, and this difficult matter will appear the easiest in the world. Once our ranks are increased in numbers, and spirit has gained in the ranks, once the battalions are home-born troops, then do you infuse into the kingdom what is lacking to it now. Homer, too, has sanctified this as the prerogative of chiefs in the words: 'Great is the wrath of divine kings.'note[Homer, Iliad 2.196.]
 Wrath, therefore, is called for against these men, and they will either till the soil in obedience to orders, even as the Messenians long ago laid down their arms and served the Lacedaemonians,note[A reference to Sparta's helots.] or they will take the same road to flight again, and announce to those beyond the river that their former gentleness no longer survives among the Romans, for that one young and nobly born is their leader
A terrible man, who might blame even the blameless.note[Homer, Iliad 11.654.]
  So be it. This assuredly is the nurture and training of a king as warrior; we will next turn our full attention to the king in peace.