This post is narrow by design, intended to make a single point about the Roman army and its commanders. In the narrative below I have constructed a chronology of the last ninety years of the empire. Each commander who is German has his name in bolded text. You will see for yourself how the Germans permeated the Roman military.
Following the death of Valens, the co-emperor in the west, Gratian, appointed a Spanish officer, Theodosius, to serve as emperor of the eastern provinces. His sixteen year reign was efficient but marked by the dubious decision to let the Goths settle south of the Danube in Roman territory (382 A.D.) under their own commanders rather than those of the empire.
Meanwhile Gratian and his commander Mallobaudes (Frank) defeated the Alamanni in May 378 A.D.
September 5-6 394 Battle of Frigidus. Theodosius defeated Eugenius, a western usurper, and his commander Arbogast (Frank). Site northeast of Aquileia.
With the death of Theodosius in 395 A.D, the empire would forever be divided into two parts. Arcadius, son of Theodosius was named emperor in the east while his other son, Honorius (age 10), was named emperor in the west.
During 395-7, the Visigoths raided the Balkans and Greece. Stilicho (half Vandal) pins them in Elis but Alaric escapes.
In 401, The Visigoths, under their king Alaric, moved from south of the Danube and attacked northern Italy, before being driven back by Stilicho, Honorius’ commander.
In 402 Ravenna became the capital of the western empire, replacing Rome.
Rome can no longer defend the provinces. Only the Italian peninsula.
In 405 German tribes attack Italy. Many cities pillaged on the way to Florence.
23 August 406 A.D. Radagaisus defeated by Stilicho at Florence and executed. Stilicho’s army includes Alani, Huns, and Goths. The survivors of the army of Radagaisus join Alaric.
In 406 Vandals, Sueves, Alans, and Burgundians crossed the frozen Rhine and attacked Gaul. This army is the remnants of the army of Radagaisus.
Arcadius died in 408 and was succeeded by his son Theodosius II who reigned for 40 years in the east.
In 408 Stilicho killed.
In 409, the Vandals settle in Spain
In 410, Alaric attacked Italy once again making it all the way to Rome where he sacked the city. Alaric dies at Bruttii.
Visigoths attack Gaul in 412.
Visigoths attack Spain in 415. Trying to protect the Spaniards from the Vandals. (Eurich) They subdue the Burgundians.
The Visigoths withdraw to Tolosa in 418.
Honorius dies in 423 to be succeeded by Valentinian III nephew of Honorius (age 6) who is controlled by Galla Placidia, daughter of Theodosius I, and the general Aetius (Scythian).
Vandals arrive in north Africa 429 invited by Boniface. They take Carthage in 439. They attack Rome in 455.
Aetius and his allied Goths defeat the Huns in 451 at Catalunian Fields.
Attila died in 453 A.D.
Ricimer (Suevic/Visigoth) essentially controls the emperorship for 16 years starting in 456. He appoints Majorian but has to overthrow him in 461. He appoints Severus III who dies in 465. The eastern emperor, Leo, nominated, Anthemius in 467, but his loss to the Vandals meant the end and Ricimer marched to Rome in 472 and killed him. Ricimer now appointed Olybrius that same year but then both men died.
Power passed to the Prince Gundobad (Burgundian) who nominated Glycerius as his puppet in 473.
The eastern emperor Zeno I dispatched a new candidate Julius Nepos who took power in 474 but was murdered by his commander the next year.
The last in line was Romulus Augustulus who abdicated in 476 ended the line of Roman emperors.
The senate told Zeno he was now emperor in the west but it was Odacer (Scirii), leader of the German mercenaries, who became king of Italy.
Note: Scirii were from the area of Poland, allied with the Huns until their fall, then the Carpathian Mountain area.
Rome in the west was now only a piece of the Italian peninsula. Gaul was controlled by the Franks, Burgundians, and Visigoths. Spain was controlled by the Visigoths and Sueves. The Vandals controlled north Africa. Roman Britain was on the way to becoming Anglo-Saxon England.
In 493, Theodoric sieges Ravenna, captures it, and puts Odacer to death. On the advice of the eastern emperor, Zeno, Theodoric becomes king of Italy, ruling over the Goths and Romans.
As I said in the beginning, a trend developed where the late Roman emperors began to use German generals to run their army. This was really the culmination of the advancement of the Germans over time. Since a substantial part of the empire was adjacent to German territory, Germans were recruited (at first enslaved) to fight for Rome. After a time, there were more Germans than Italians in the Roman army so all that remained was for the commanders to become Germans too.
This development blurs the definition of the “end of the empire” because if the Germans were commanding a Roman army of Germans, weren’t they essentially controlling Rome? The emperors from Honorius on were puppets.
I also want to mention once again the attitude of the Goths toward Rome, which was essentially respectful. The Goths wanted to settle in more fertile areas south of the Alps and sought peaceful coexistence with the empire. It was only when Rome offended them repeatedly (as in 410 A.D.) that they became more militant. The Goths fought for Rome many more times than they fought against Rome.
The Vandals, of course, are another story – we named wanton destruction after them.