Herodian (late second, first half third century): Greek historian, author of a History of the Roman Empire since the Death of Marcus Aurelius in which he describes the reign of Commodus (180-192), the Year of the Five Emperors (193), the age of the Severan dynasty (211-235), and the Year of the Six Emperors (238).
The translation was made by Edward C. Echols (Herodian of Antioch's History of the Roman Empire, 1961 Berkeley and Los Angeles) and was put online for the first time by Roger Pearse (Tertullian.Org). The version offered on these pages is hyperlinked and contains notes by Jona Lendering.
[6.7.1]  Alexander did not believe that Persian affairs would remain permanently quiet and peaceful, but he did think that the barbarian had provided him with a temporary respite from campaigning. The barbarian army, once disbanded, was not easily remustered, as it was not organized on a permanent basis. More a mob than a regular army, the soldiers had only those supplies which each man brought for himself when he reported for duty. Moreover, the Persians are reluctant to leave their wives, children, and homeland.
[6.7.2] Now unexpected messages and dispatches upset Alexander and caused him even greater anxiety: the governors in Illyria reported that the Germans [the Alamans] had crossed the Rhine and the Danube rivers, were plundering the Roman empire, and with a huge force were overrunning the garrison camps on the banks of these rivers, as well as the cities and villages there. They reported also that the provinces of Illyricum bordering on and close to Italy were in danger.
[6.7.3] The governors informed the emperor that it was absolutely necessary that he and his entire army come to them. The revelation of these developments terrified Alexander and aroused great concern among the soldiers from Illyricum, who seemed to have suffered a double disaster; the men who had undergone many hardships in the Persian expedition now learned that their families had been slaughtered by the Germans. They were naturally enraged at this, and blamed Alexander for their misfortunes because he had betrayed affairs in the East by his cowardice and carelessness and was hesitant and dilatory about the situation in the North.
[6.7.4] Alexander and his advisers, too, feared for the safety of Italy itself. They did not consider the Persian threat at all similar to the German. The fact is that those who live in the East, separated from the West by a great continent and a broad sea, scarcely ever hear of Italy, whereas the provinces of Illyricum, since they are narrow and very little of their territory is under Roman control, make the Germans actually neighbors of the Italians; the two peoples thus share common borders.
[6.7.5] Although he loathed the idea, Alexander glumly announced his departure for Illyria.note[Herodian does not mention Alexander's return to and triumph in Rome. He also makes a strange topographical error: the emperor stayed in Mainz in Germania Superior, not in Illyricum.] Necessity compelled him to go, however; and so, leaving behind a force which he considered strong enough to defend the Roman frontiers, after he had seen to the forts and the walls of the camps with greater care and had assigned to each fort its normal complement of troops, the emperor marched out against the Germans with the rest of his army.
[6.7.6] Completing the journey quickly, he encamped on the banks of the Rhine and made preparations for the German campaign. Alexander spanned the river with boats lashed together to form a bridge, thinking that this would provide an easy means of crossing for his soldiers. The Rhine in Germany and the Danube in Pannonia are the largest of the northern rivers. In summer their depth and width make them easily navigable, but in the cold winters they freeze over and appear like a level plain which can be crossed on horseback.
[6.7.7] The river becomes so firm and solid in that season that it supports horses and men. Then those who want drinking water do not come to the river with pitchers and bowls; they bring axes and mattocks and, when they have finished chopping, take up water without using bowls and carry it in chunks as hard as rock.
[6.7.8] Such is the nature of these rivers. Alexander had brought with him many Moorish javelin men and a huge force of archers from the East and from the Osrhoenian country, together with Parthian deserters and mercenaries who had offered their help; with these he prepared to battle the Germans. The missile men were especially troublesome to the Germans: the Moorish hurl their javelins from a distance and attack and retreat nimbly, while the archers, far removed from their targets, easily fire their arrows into the bare heads and huge bodies of the Germans; but when the Germans attacked at full speed and fought hand to hand, they were often the equal of the Romans.
[6.7.9] Alexander was thus occupied with these matters. He thought it wise, however, to send an embassy to the Germans to discuss the possibilities of a peaceful settlement. He promised to give them everything they asked and to hand over a large amount of money. The avaricious Germans are susceptible to bribes and are always ready to sell peace to the Romans for gold. Consequently, Alexander undertook to buy a truce rather than risk the hazards of war.
[6.7.10] The soldiers, however, were not pleased by his action, for the time was passing without profit to them, and Alexander was doing nothing courageous or energetic about the war; on the contrary, when it was essential that he march out and punish the Germans for their insults, he spent the time in chariot racing and luxurious living.