Alexander's generals, including Seleucus, divided his conquests, including the Persian Empire, between them. In 209 BC, the Seleucid Antiochus III rules "Syria," comprising Syria, Persia, Mesopotamia and parts of Turkey and the Mediterranean seaboard. Sixty years previously, Parthia (northeastern Persia) and Bactria (in northern Afghanistan) had declared themselves independent kingdoms. Antiochus has reconquered Parthia and now besieges Bactra, the city of Euthydemus, King of Bactria.
Thus the history. Manson Everard, Unattached agent of the Time Patrol, must enter Bactra just before the siege starts because he hopes that the Exaltationist time criminals have followed planted evidence that, by changing the outcome of a battle outside Bactra, they can gain control of subsequent history. Everard hopes to arrest the last of the Exaltationists.
Yet again, Poul Anderson seamlessly melds historical fiction with science fiction.