[1] He began his reign by killing two of his brothers, but the third, later called Zipoetes II, rebelled against him and managed to maintain independent sovereignty over a considerable part of Bithynia for some time.

It was against his brother rather than his foreign enemies that Nicomedes now called in more powerful auxiliaries and formed an alliance with the Celts, who had arrived on the other side of the Bosphorus under Leonnorius and Lutarius and were at this time engaged in the siege of Byzantium in 277 BC.

After providing them with the means to cross into Asia, where they founded Galatia, he turned the weapons of his new auxiliaries against Zipoetes II, whom he defeated and killed, thus reuniting the whole of Bithynia under his rule.

It seems, however, that Nicomedes was left in the undisturbed possession of Bithynia, which he ruled from this time until his death, and which rose to great power and prosperity during his long and peaceful reign.

Following in the footsteps of his father and so many other Greek rulers of Asia, he decided to perpetuate his own name by founding a new capital, and the site he chose, near the Megarian colony of Astakos, was so wisely chosen that the city of Nicomedia remained one of the richest and most prosperous in Anatolia for more than six centuries.