The Seleucid phalanx, the backbone of the army, was mobilized for the big wars primarily from among the military settlers of Greco-Macedonian descent who were called to the colours in time of need. However, the possibility of utilizing military settlers was not unlimited. As farmers, they could not leave their settlements for very long periods, and their mobilization itself was a lengthy process because of the great distances between settlements, and between the settlements and the governmental centres and the battle sites. To fulfil basic military needs and supply manpower for ordinary guard duties and police functions as well as extinguish small local conflagrations, the Seleucid kings had a Royal Guard. Like all royal armies even to the present day, the Guard was at the disposal of the king at all times in peace and in war, and in battle the kings always took their place in one of the Royal Guard units. As might be expected, most of the Seleucid Royal Guard were foot soldiers, only a minority being mounted. We shall deal here solely with the infantry Royal Guard. This investigation can illuminate not only the matter of the regular arrangements for the security of the Seleucid empire, but also the way that the military settlements were organized, and in particular the manner in which the military capability of the heavy infantry forces in general was preserved.

Among the various units of the Seleucid army mentioned in the sources, three – the argyraspides, hypaspists and peltasts – refer at different times to units of the Royal Guard in the armies of Alexander, the Diadochs, and the Antigonid dynasty.