The Political landscape
To appreciate how the migrations of the early 5th century were able to move through and settle within the empire it would do well to look at events that shaped the political scene in the west following the reign of Constantine I. The list highlights the major incursions and civil confrontations after 310 AD to the Rhine crossing of 406 AD. Most of the information is gleaned from the website Fectio.
Between the engagements against contenders for the throne Constantine did fight the Franks (310), Sarmatians (322), the Goths (323), his successor, Constantine II defeats the Alamanni (328), the Goths (332), and the Sarmatians (334). In some cases the defeated were relocated such as the 30,000 Sarmatians sent to Italy and Gaul.
Constans continues this policy and settles the Salian Franks in former Batavia (342). Civil war ensues (350) as Magnentius is proclaimed emperor at Autun. The Alamanni, led by Chnodomar, take advantage of the situation to defeat Decentius (352). Constantius defeats Magnentius and can now confront the Alamanni and defeats them at Campi Canini (355). Julian continues the campaign against the Alamanni and defeats them at Strasbourg (357). Constantius campaigns along the Danube against the Sarmatian and Quadi (358) while Julian fights the Franks and Alamanni re-settling the Salian Franks as foederati in Toxandria (358).
The Alamanni cross the frozen Rhine (366) but are defeated at Scarponna. The invasion of Britain by the Picts, Scotti and Atacotti (367) is neutralised while the Franks and Saxons attack the Gallic coast. Britain is sent Jovinus to resolve the state of anarchy, lacking enough troops he returns to Gaul. The Alamanni cross the Rhine and plunder Mainz.
Valentinian and Gratian defeat the Alamanni (368) at Solicinium (Schwetzingen). Comes Flavius Theodosius Sr. begins campaigning in Britain and pacifies the south. Theodosius rallies the scattered units in Britain and puts down the rebellion of Valerius (369). Theodosius is promoted to magister equitum. Frithigern becomes king of the Visigoths.
In retribution for their attacks on the comes Nannienus, the Saxons in northern Gaul are defeated by Severus (magister peditum) near Deuso (370). Theodosius Sr. campaigns against the Alamanni marching from Raetia into Germania. Theodosius continues campaigning this time against the Sarmatians (372) and Valentinian sends Alamannic foederati to Britain as reinforcements.
Theodosius is sent to Africa to put down the usurper Firmus (373) and the Huns enter Europe after defeating the Alans at Tanais. Theodosius Sr. is, under questionable circumstances, executed in Carthago. Gratian mobilizes an army to assist Valens with the situation against the Goths (377). The Alamanni cross a frozen Rhine (378) to invade the west. Gratian campaigns against them in the Black Forest. The defeat at Adrianople opens the Danube line for other invasions. Flavius Theodosius Jr. pushes the Sarmatians back across the Danube (378).
Elevated to Augustus in the East, both he and Gratian concentrate the efforts and deal with the Goths (380,382). Comes Britanniarum M. Maximus defeats the Picts and Scots.
Maximus claims the purple in the spring (383) and crosses to Gaul, Gratian campaigns in Raetia before moving against Maximus. Gratian’s army defects to Maximus and kill Gratian. Maximus agrees to become Augustus of the West (384) while Valentinian II rules the Middle Empire (Italy, Illyria, Africa). Maximus makes his son Victor Augustus reneging on the agreement. Theodosius occupied with the Goths and Persians makes no move against Maximus.
Maximus invades Italy (387) and Valentinian retreats to Thessalonica. Colonia (Cologne) is threatened by the Frankish Kings Gennobaudes, Markomer and Sunno (388). The field army commanded by Nannienus and Quintinus counterattack and are victorious at Silva Carbonnaria (Kohlenwald). Quintinus is later defeated at Novaesium (Neuss). Theodosius and Valentinian II defeat Magnus Maximus at Emona, Siscia (Sisak) and Poetovio (Pettau). Maximus surrenders and is executed.
Theodosius reorganises the army and appoints Arbogast as magister militum of the west. Valentinian is made emperor of the west (389). The Franks again invade Gaul which leads to a feodus (treaty) between them and Valentinian.
Arbogast campaigns across the Rhine (391) against the Bructeri and the Chamavi. A law is placed by Theodosius allowing provincials to defend themselves against rogue soldiers.
Arbogast no longer acknowledges Valentinian and is later found dead, an alleged suicide (392). Arbogast sets Flavius Eugenius (magister scrini) to the throne. Arbogast campaigns against the Franks near Colonia. Theodosius places his second son Flavius Augustus Honorius as Augustus of the West (393).
Theodosius campaigns against Eugenius and Arbogast and defeats both at the Frigidus (394). Beheading Eugenius followed by the suicide of Arbogast, Theodosius is now sole emperor with his sons Arcadius and Honorius as co-regents in the East and West.
Theodosius dies (395) splitting the empire, Arcadius rules in the East and Honorius in the West.
Stilicho (magister peditum) contains Alaric in Thessalia and marches back to Rome. The Gallic prefecture is transferred from Trier to Arelate (Arles).
Stilicho renews treaties with several Germanic kings located along the Rhine (396). Stilicho musters forces to confront Alaric, but is later recalled (397). Alaric is made magister militum per Illyrium by Eutropius (Arcadius ’magister officiorum). The comes Africae Gildo separates from the Western Empire with the support of Arcadius and Eutropius. Mascezel campaigns against Gildo and defeats him at Ammaedara (Haidra) and Theveste (Tebessa). Gildo is executed (398).
Stilicho marches against the invading Vandals and Alans in Raetia (401) and Alaric invades Italy in November. Alaric marches on Milan (402) and Stilicho gathers forces from the border regions to defend Italy. Stilicho defeats Alaric at Pollentia (Pollenza) and at Verona, but allows Alaric to escape. Honorius moves the court from Milan to Ravenna. Alaric leaves Italy (403).
Germanic tribes lead by Radagais invades Italy (405). Stilicho besieges Radagais near Faesulae (Fiesole) capturing him (406). An invading confederation of Siling and Asding Vandals (King Godigisel), Alans (Kings Goar and Respendial), Burgundians and Suevi cross a frozen Rhine into Gaul.