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In the Pontic Steppe and Europe

During the 4th and 3rd centuries BC, the centre of Sarmatian power remained north of the Caucasus and in the 3rd century BC the most important centres were around the lower Don, Kalmykia, the Kuban area, and the Central Caucasus.

During the end of the 4th century BC, the Scythians, the then dominant power in the Black Sea Steppe, were militarily defeated by the Macedonian kings Philip II of Macedon and Lysimachus in 339 and 313 BC respectively. They experienced another military setback after participating in the Bosporan Civil War in 309 BC and came under pressure from the Thracian Getae and the Celtic Bastarnae. At the same time, in Central Asia, following the Macedonian conquest of the Achaemenid Empire, the new Seleucid Empire started attacking the Sakā and Dahā nomads who lived to the north of its borders, who in turn put westward pressure on the Sarmatians. Pressured by the Sakā and Dahā in the east and taking advantage of the decline of Scythian power, the Sarmatians began crossing the Don river and invaded Scythia and also migrated south into the North Caucasus.

The first wave of westward Sarmatian migration happened during the 2nd century BC, and involved the Royal Sarmatians, or Saioi (from Scytho-Sarmatian *xšaya, meaning "kings"), who moved into the Pontic Steppe, and the Iazyges, also called the Iaxamatai or Iazamatai, who initially settled between the Don and Dnieper rivers. The Roxolani, who might have been a mixed Scytho-Sarmatian tribe, followed the Iazyges and occupied the Black Sea steppes up to the Dnipro and raided the Crimean region during that century, at the end of which they were involved in a conflict with the generals of the Pontic king Mithridates VI Eupator in the Bosporan Chersonesus, while the Iazyges became his allies.

That the tribes formerly referred to by Herodotus as Scythians were now called Sarmatians by Hellenistic and Roman authors implies that the Sarmatian conquest did not involve a displacement of the Scythians from the Pontic Steppe, but rather that the Scythian tribes were absorbed by the Sarmatians. After their conquest of Scythia, the Sarmatians became the dominant political power in the northern Pontic Steppe, where Sarmatian graves first started appearing in the 2nd century BC. Meanwhile, the populations which still identified as Scythians proper became reduced to Crimea and the Dobruja region, and at one point the Crimean Scythians were the vassals of the Sarmatian queen Amage. Sarmatian power in the Pontic Steppes was also directed against the Greek cities on its shores, with the city of Pontic Olbia being forced to pay repeated tribute to the Royal Sarmatians and their king Saitapharnes, who is mentioned in the Protogenes inscription along with the tribes of the Thisamatae, Scythians, and Saudaratae. Another Sarmatian king, Gatalos, was named in a peace treaty concluded by the king Pharnaces I of Pontus with his enemies.

Two other Sarmatian tribes, the Siraces, who had previously originated in the Transcaspian Plains immediately to the northeast of Hyrcania before migrating to the west, and the Aorsi, moved to the west across the Volga and into the Caucasus mountains' foothills between the 2nd to 1st centuries BC. From there, the pressure from their growing power forcing the more western Sarmatian tribes to migrate further west, and the Aorsi and Siraces destroyed the power of the Royal Sarmatians and the Iazyges, with the Aorsi being able to extend their rule over a large region stretching from the Caucasus across the Terek–Kuma Lowland and Kalmykia in the west up to the Aral Sea region in the east. Yet another new Sarmatian group, the Alans, originated in Central Asia out of the merger of some old tribal groups with the Massagetae. Related to the Asii who invaded Bactria in the 2nd century BC, the Alans were pushed west by the Kangju people (known to Graeco-Roman authors as the Ιαξαρται Iaxartai in Greek, and the Iaxartae in Latin) who were living in the Syr Darya basin, from where they expanded their rule from Fergana to the Aral Sea region.

The hegemony of the Sarmatians in the Pontic Steppe continued during the 1st century BC, when they were allied with the Scythians against Diophantus, a general of Mithradates VI Eupator, before allying with Mithradates against the Romans and fighting for him in both Europe and Asia, demonstrating the Sarmatians' complete involvement in the affairs of the Pontic and Danubian regions. During the early part of the century, the Alans had migrated to the area to the northeast of the Lake Maeotis. Meanwhile, the Iazyges moved westwards until they reached the Danube, and the Roxolani moved into the area between the Dnipro and the Danube and from there further west. These two peoples attacked the regions around Tomis and Moesia, respectively. During this period, the Iazyges and Roxolani also attacked the Roman province of Thracia, whose governor Tiberius Plautius Silvanus Aelianus had to defend the Roman border of the Danube. During the 1st century BC, various Sarmatians reached the Pannonian Basin, with the Iazyges passing through the territories corresponding to modern-day Moldavia and Wallachia before settling in the Tisza valley, by the middle of the century.

Although the Sarmatian movements stopped temporarily during the 1st century BC due to the rise of the Dacian kingdom of Burebista, they resumed after the collapse of his kingdom following his assassination and in 16 BC. Lucius Tarius Rufus had to repel a Sarmatian attack on Thracia and Macedonia, while further attacks around 10 BC and 2 BC were defeated by Gnaeus Cornelius Lentulus.

Meanwhile, other Sarmatian tribes, possibly the Aorsi, sent ambassadors to the Roman emperor Augustus, who tried to establish a diplomatic accommodation with them. During the 1st century AD, the Siraces and Aorsi, who were mutually hostile, participated in the Roman–Bosporan War on opposite sides: the Siraces and their king Zorsines allied with Mithridates III against his half-brother Cotys I, who was allied with Rome and the Aorsi. With the defeat of Mithridates, the Siraces were also routed and lost rulership over most of their lands. Between 50 and 60 CE, the Alans had appeared in the foothills of the Caucasus, from where they attacked the Caucasus and Transcaucasus areas and the Parthian Empire. During the 1st century AD, the Alans expanded across the Volga to the west, absorbing part of the Aorsi and displacing the rest, and pressure from the Alans forced the Iazyges and Roxolani to continue attacking the Roman Empire from across the Danube. During the 1st century AD, two Sarmatian rulers from the steppe named Pharzoios and Inismeōs were minting coins in Pontic Olbia.

The Roxolani continued their westward migration following the conflict on the Bosporan Chersonesus, and by 69 AD they were close enough to the lower Danube that they were able to attack across the river when it was frozen in winter, and soon later they and the Alans were living on the coast of the Black Sea, and they later moved further west and were living in the areas corresponding to modern-day Moldavia and western Ukraine.

The Sarmatian tribe of the Arraei, who had had close contacts with the Romans, eventually settled to the south of the Danube river, in Thrace, and another Sarmatian tribe, the Koralloi, were also living in the same area alongside a section of the Scythian Sindi.

During the 1st and 2nd centuries AD, the Iazyges often bothered the Roman authorities in Pannonia; they participated in the destruction of the Quadian kingdom of Vannius, and often migrated to the east across the Transylvanian Plateau and the Carpathian Mountains during seasonal movements or for trade.

By the 2nd century AD, the Alans had conquered the steppes of the north Caucasus and of the north Black Sea area and created a powerful confederation of tribes under their rule. Under the hegemony of the Alans a trade route connected the Pontic Steppe, the southern Urals, and the region presently known as Western Turkestan. One group of the Alans, the Antae, migrated north into the territory of what is presently Poland.