The history of the Seleucid army embraces a period of more than 200 years during which the kings carried on campaigns of different kinds against competitors in the Hellenistic world such as Lysimachus and the Houses of Ptolemy and Antigonus, barbarian invaders like the Galatians, rebels like Hierax, MoIon, Achaeus, and Timarchus, subject nations like the Jews and the Persians, and the two rising powers, the Romans in the west and the Parthians in the east, who finally divided the Seleucid domain between them. Some of these conflicts had far-reaching historical implications: the victory over Antigonus destroyed all hope of unifying the Hellenistic world and weakened its ability to meet challenges; the battles at Thermopylae and Magnesia heralded the shift in power in the Mediterranean from east to west; and the abortive attempt to suppress the Jewish religion deeply influenced the evolution of new religious beliefs and standards which left their mark on western civilization for many centuries.

The two outstanding features of the Seleucid army were, on the one hand, its quantitative and qualitative superiority over other Hellenistic armies, and, on the other, its inability to withstand the Roman army. In order to understand the first phenomenon, one must examine the organization of the Seleucid army, trace its sources of manpower, and ascertain the way in which its availability and military standards were maintained. These factors, in conjunction with the tactics and operations of the armies on the battlefield itself, may reveal the defects of the best of the Hellenistic armies when compared with the Roman legions.