No migration period (Abrittus)

Migrations have happened throughout the entire pre-history and history of humankind. They continue to happen today.

Nevertheless, some periods of time experience very significant migratory movements with far-reaching historical consequences. From a eurocentric perspective, one of these periods has come to be called the "Migration Period". It is usually taken to mean the time span of the 4th to 7th centuries of OTL, when Huns, Awars, Alani, Goths, Franks, Suebians, Vandals and Langobards migrated and pushed each other Westwards, bringing about the collapse of the Western Roman Empire and the order of Late Antiquity and the establishment of Roman-Germanic feudal kingdoms and the transition to medieval structures.

The above-mentioned migratory movements in OTL are only the European section of a larger picture. The initial impulse which got the ball rolling needs yet to be determined: climatic change affecting Central Asia? Chinese military campaigns against nomads? Military advances (the stirrup? better crossbows? larger horses?) by some the steppe nomads?

Either way, the migratory movements began in Eastern Central Asia: in the Gansu corridor, the Tarim basin, Dsungaria and Sogdiana. From Eastern Central Asia, they spilled Southwards and Westwards (because to the North, there were only endless forests). In OTL, Kidarites, Xionites and later Hephtalites exerted pressure on the Sassanid Empire and brought down the Gupta Empire in Northern India. Those who moved Westwards encountered other nomads in the Caspian and Pontic steppes. Overcrowding caused newly composed second, third etc. waves - led by "the Huns" or later "the Awars", labels which describe heterogeneous confederacies forming and transforming for and during migratory movements and the military events brought about by them. Migration, displacement and militarisation caught one group after the other in their avalanche and maelstrom. Repeatedly, groups attempted to build new state structures in the cultural areal where statehood was the norm, and repeatedly, Migration Period states like the Ostrogothic Kingdom in the Balkans, Magna Hungaria or Great Bulgaria were overthrown by yet another wave of migration. While most of Western, Central and Southern Europe were spared from further migratory waves from the 6th century onwards, the Balkans and Eastern Europe as well as Central Asia experienced many more Westward mass migrations for more than half a millennium.

In this timeline, the Roman Republic and the Sassanid Empire keep peace with each other. While Rome is politically much more stable than in OTL, the Sassanids are militarily much more powerful than in OTL. The Sassanids prevent the Southward migration from the 4th century on and commit a genocidal bloodbath among the nomads who had invaded Bactria, Transoxania and Sogdia. The Romans, in turn, stop the Westward migration a few decades later and commit an equally gruesome genocide in the Pontic steppe.

Thus, Romans and Sassanids not only saved themselves from imminent danger, but bought themselves two centuries of peace. The depopulation of the Western half of the Central Asian steppe dramatically deflated population pressure North of the Empires. For two centuries, new migratory movements found plenty of unoccupied space and came to a halt.

Here is a short table comparing the fate and location of various groups in OTL vs. Abrittus TL: