Alexander's campaigns created a world that his successors spent fifty years fighting over and never succeeded in unifying. The Diadochi (successor) wars produced three major kingdoms that dominated the Hellenistic period: the Ptolemaic Kingdom in Egypt, the Seleucid Empire in Asia, and the Antigonid Kingdom in Macedon and Greece.

The Seleucid Empire is the most underappreciated of the three. At its maximum extent it controlled territory from Anatolia to Bactria (Afghanistan) — the largest political unit in the ancient world west of China. The challenge of governing this territory, with its enormous cultural and linguistic diversity, led the Seleucids to develop administrative practices that were genuinely innovative: local temples and religious institutions were maintained and patronized as tools of legitimacy, local elites were incorporated through marriage alliances, and the imperial court was deliberately multicultural.

The gradual contraction of Seleucid power — Parthian expansion in the east, Roman pressure in the west, internal dynastic conflicts — is one of the most important geopolitical processes of the 2nd-1st centuries BC. By the time Rome eliminated the last Seleucid ruler in 64 BC, the empire had already lost most of its eastern territories. The Parthian Empire, which absorbed the Iranian plateau, preserved elements of Seleucid and Achaemenid administrative practice under a new Parthian ruling house.

The cultural legacy of the Seleucid Empire — Hellenistic culture embedded in a Near Eastern social context — created the world that early Christianity emerged from. The New Testament was written in Greek because Hellenistic culture had made Greek the common language of the Eastern Mediterranean for three centuries.