Migrants from Thrace begin to settle in northwestern Anatolia late in the second millennium, perhaps earlier than the semi-legendary Trojan War of the late thirteenth or early twelfth century, as the result of a mass movement of peoples from Macedonia.

The chief Megaran colony is Astacus (modern Izmit) in northwestern Asia Minor.

Nicomedes of Bithynia, in imitation of his father, and so many others of the Greek rulers of Asia, determines to perpetuate his own name by the foundation of a new capital, placed by Eusebius in 264 BCE.

The site that he chooses, in the immediate neighborhood of the Megarian colony of Astakos, is so judiciously selected that the city of Nicomedia will continue for more than six centuries to be one of the richest and most flourishing in Anatolia.

Nicomedes' second wife Etazeta becomes ruler on behalf of her infant sons after his death in 255 BCE.

The grown-up Ziaelas, son of Nicomedes and Ditizele and thus excluded from the throne, had previously fled to Armenia and taken refuge at the court of King Arsames I in Sophene.

On his father's death, he immediately endeavors to regain his rights by force and returns, aided by some Galatians.

Although Etazeta is supported by neighboring cities and Antigonus II Gonatas, Ziaelas rapidly conquers first part, then all of Bithynia, forcing Etazeta and her sons to escape to Antigonus' court in Macedonia about 254 BCE.

Zielas of Bithynia, like his father and his grandfather, also founded a new city named after him, Ziela, but the location of the city is unknown.

He is succeeded by his son Prusias I about 228 BCE after being killed by the Galatian Gauls.

Seleucus now invades Anatolia, initiating the so-called War of the Brothers (239-236).

Prusias fights a war against Byzantium in 220 BCE, then defeats the Galatians who his grandfather, Nicomedes I, had invited across the Bosporus.

Prusias I has expanded the territories of Bithynia in a series of wars against Attalus I of Pergamon and Heraclea Pontica on the Black Sea.

Philip V of Macedon grants him the ports of Keios and Myrleia in 202, which he renames Prusias and Apameia respectively.

Hannibal's most famous achievement had been at the outbreak of the Second Punic War, when the Carthaginian military commander and tactician marched an army, which included war elephants, from Iberia over the Pyrenees and the Alps into northern Italy.

He eventually takes refuge with Prusias of Bithynia, who at this time is engaged in warfare with Rome's ally, King Eumenes II of Pergamon.

He serves Prusias in this war, and, in one of the victories he gains over Eumenes at sea, it is said that he had had cauldrons of snakes thrown into the enemy vessels.

Pharnaces of Pontus, without waiting for the return of his ambassadors, decides in the spring of 181 to attack both Eumenes and Ariarathes IV of Cappadocia and therefore invades Galatia with a large force.

Eumenes, allied with Prusias II Cynegus, who had succeeded his father Prusias as king of Bithynia on the latter’s death in 182, leads an army to oppose him, however, hostilities are soon suspended following the arrival of Roman deputies, who have been appointed by the Roman Senate to inquire into the matters in dispute.

Negotiations take place at Pergamon but are inconclusive, with Pharnaces' demands being rejected by the Romans as unreasonable.

As a consequence, the war between Pontus and Pergamon and her allies is renewed.