Ascension to the Throne
Horribly marred by the attack, with a massive scar along his face, he gains the agnomen Cicatrix; Anthemius the Scarred. Shaken by the brutality of the attack, Anthemius began devoting considerable time to the study of military theory. As Magister Militum, he realizes that the fall of Rome is inevitable. His rise to the Imperial Throne occurs as in OTL, with Leo the Thracian, Eastern Roman Emperor, declaring him Caesar of Western Rome, only doing so in 465, having had his rival killed by the same mercenaries. Emperor Anthemius begins a remodeling of his plan to escape Rome's potential fall on a far grander scale.
He began carefully collecting tax money and confiscating treasure from across the entire Empire in order to fund his massive project, which he dubbed "Romae Foris", Rome Abroad.
He formed five new Legions, Cicatrix I through Cicatrix V, recruiting as often as possible from local Italian populations, under the pretense of forming a "Roman Defense", the Praesidium Romanum, to defend Rome from Mauraders. Befitting their ostensible role of static defense of Rome, they were equipped more heavily than He also retooled the Praetorian Guard to be led by the same mercenaries that saved his life on the Danubian Frontier.
The prescient Emperor needed a location from which he could survive the potential fall of Rome; a geographically isolated location, but one with sufficient resources to support five legions of Roman Infantry. He settles upon the former province of Britannia, the land having lapsed into warring fiefdoms in fifty-five years of Roman absence.
In 467, he ignored shared plans with the Eastern Empire to invade Vandal-held North Africa, instead rerouting the fleet, along with hundreds of other ships, in order to transport the Legio Cicatrix II, along with their families. This fleet comprised the majority of Western Roman Naval Power at the time; a fact that alarmed Leo the Thracian. The Eastern Roman Emperor traveled to meet with Anthemius Cicatrix; demanding to know why Legio Cicatrix II had set sail with most of the Western Empire's navy and a significant portion of its civilian shipping.
Meanwhile, Legio Cicatrix III and IV have begun marching across Gallia, with their families joining the normal train of camp followers. The massive, 120 000 person march across Gallia is made worse by the Legions' habit of removing wealth from the province as they went, in order to prepare for their time in Britannia.
Leo the Thracian is made aware of this, and attempts to arrest Anthemius. He is stopped by the newly reformed Praetorian Guard, who execute Leo I and leave Rome with Anthemius and Legions I and V. They essentially sack Rome when they do; taking with them much of what wealth remained in the city. They move quickly to catch up with Legions III and IV, facing significant hostility from local populations. Anthemius cleverly deflects this, stating that Legio III and IV are acting on their own accord; stating that his intentions were to hunt down the rebels; his lie pacifying locals enough to allow him to pass.
In the Eastern Roman Empire, Zeno; son-in-law of Leo the Thracian, returns from a campaign against the Goths to hear of his Leo's death. His infant son is heir apparent; Zeno acts as regent in his stead. He takes the army he lead against the Goths, as well as Leo's brother-in-law Basiliscus' army (intended to campaign against the Vandals) and heads to Rome to kill Anthemius. He finds the city abandoned and in chaos, following the "Fourth Sacking of Rome of 468"
The combined army settles in to restore order to Rome; they are aided by the General Marcellinus, the magister militum of Dalmatia and part of the planned campaign against the Vandals. He too is upset by Cicatrix's flight; the five legions having robbed Rome of the bulk of its military and financial assets, and the campaign's cancellation hurting his prestige.
Zeno took command of the situation; he invited Basiliscus to become co-regent of the Eastern Empire with him, until Leo II was old enough; Marcellinus was married to Leonitia, Zeno's sister-in-law, and given the title of Magister Roma, in concert with his responsibilities in Dalmatia. They three formed a new Triumvirate, in order to better control the Western and Eastern Empires.
Zeno returned to govern in the East, sending Basiliscus and his fleet to pursue Anthemius. He leaves Marcellinus with the unenviable task of maintaining control in Rome.
To accomplish this, Marcellinus attempts the opposite. He withdraws the remainder of non-Italian troops from Italy, sending them to engage the Visogoths. The campaign goes horribly; the Visogoths sacking Rome in retaliation. Marcellinus uses this to rally the people's of Italy, reinvigorating the spirit that allowed for Roman Success in the early days of the Republic. He forms several new local legions, following in the same vein as Anthemius, but this time holding true to his word that they would be for defense of Rome. He builds a series of walls, watchtowers and forts along the Southern Italian Coast, placing a Legion in Sicily to deploy against Vandal predation. He also forms a series of dozens of forts and manned passes in Cisalpine Gaul, to guard against future invasion by Visogoths. He links his territories of Italy and Dalmatia via a string of new forts along the road from Rome to Dalmatia.
In 469, Anthemius and Legions I, III, IV and V arrive in the newly reconquered Britannia. Legio II had conquered south-western Britannia, absorbing Cerniw and Dumnonia. They reform the now patchwork collection of independent farms into the old Latifunda system, freeing up manpower to join the Legions. The five Legions, enhanced by two Auxillary Legions from the local "provinces", and the Praetorian Guard, swiftly conquer Durotrigia and Atrebatia. Gewissae joins willingly; the now eleven legion strong army facing off against an alliance of Glouvia, Gwent, Glywysing, and Luitcoyt. The confederation does not last long; falling in 471.
The Roman advance is halted by the arrival of Basiliscus and his two Legions of Roman troops. They are met at the Battle of [Need a name] by Legio III Cicatrix and two Auxillary Legions. They shatter Basiliscus' men; the Auxillaries feigning flight, drawing the two Legions down on III Cicatrix. Legio III allows themselves to be boxed in; the Basiliscus' forces surrounding them. The Auxillaries then reappear, pincering the two Eastern Legions. The sudden shift in the favor of battle demoralizes the army; Basiliscus is captured and a third of his men are killed. The remainder opt to join Anthemius' army; each is replenished in numbers by local men, and the two Legions are headed by Anthemius' sons.
A confederation of Dyfed, Ceredigion, Gwynedd, and Powys attempt to attack the Northern Roman Empire. The Empire defeats the Confederation, and absorbs them into the Empire in 470 AD.
While the Byzantine Empire and Marcellinus' territory are under control, the rest of the Western Roman Empire falls to barbarian tribes far more swiftly. In the aftermath, the Huns control the northern Balkans, the Ostrogoths are displaced to Southern Germania, the Alemanni consolidate their hold on Central Europe, and the Franks overwhelm Gallia, allying with the Burgundians. The remnants of the Gallic Army flee to Briton settlements in northwestern Gallia; allying with them for mutual protection. The Visogoths invade and capture Hispania, and join the Franco-Burgundian Alliance.
The Visogothic Kingdom forms in what became Aquitaine in OTL; remaining at peace with the Franks, Burgundians and Vandals, but only barely.
The Tri-Barbarian Alliance invades Italy; a Franco-Burgundian force attacking Cisalpine Gaul; the Vandals landing in Southern Italy. Marcellinus leads a force in the north; reinforcements from Dalmatia face the Vandals. After the Franks and Burgundians are repulsed, Marcillenus proceeds to eliminate the Vandal Army, forcing the Vandals to sue for peace.
In the Byzantine Empire, the co-regent Zeno heared of Basiliscus' capture; secretly thankful that the potential threat to his power was gone. He declares himself sole regent once again; and then had Marcellinus and Pope Simplicus declare him the rightful Emperor; his son now heir apparent. Zeno is forced to forgo plans for vengeance on Anthemius in favor of defending his Empire against Sassanid predation.
The Northern Roman Empire
Taking on the name the Northern Roman Empire, Anthemius' army set about campaigns to conquer the island of Britannia. They began by recapturing a large swath of territory along Hadrian's Wall, cutting of Caledonia from the rest of Britannia. Over the course of the next twelve years, the other petty kingdoms of Britannia fell under the Northern Roman Yoke.
In 483, Leo II became Emperor, his father Zeno serving as his heir. In the NRE, Anthemius died, leaving his Empire in the hands of his son, Marcian; Marcian began a campaign north from the wall, bringing with him seven Legions. He defeated a Pictish Confederation in 483; and after eight years of campaigning and careful alliances with minor Pictish warlords, in 491, had conquered the entirety of Caledonia.
In Western Rome, Marcinellus died, his empire also falling to his son; an adopted Italian named [need a name]
The Empire stabilized after that, enjoying the "Pax Marciana". His son, born on Britannia, succeded him in 510. Emperor Anthemius II Britannicus began a campaign into Ireland in 511, but was forced to retreat due to the expense of maintaining so many legions in peacetime. He began a succession of Emperors that chose to develop the local infrastructure over new legions; instead keeping only five Legions; Cicatrix I-V, and placing one in each corner of the Empire.