The River-Plains Nation consists of a collection of chiefdoms occupying much of the territory between the major rivers in the north, and the mountains in the south. They have uneasy relations with the Malkh kingdoms (and Tuaxi, to a lesser extent), whom they have partly displaced over several centuries.

The Aspawatam are semi-nomadic horsemen, famed as charioteers. Their invasion of the River-Plains required several decades to complete, though their victory was more-or-less assured following their seizure of the port at Tanupar. While they could never attempt to control access to the Western Sea, their siege succeeded largely because Tanupar depended on trade revenue, and the Aspawatam choked off the flow of bronze and raw zinc from the mines and foundry cities in the far Northeast. Further, Tanupar was never reinforced; the Achaiói and Danaói rulers sent for help, but never received it and it has been assumed that this was caused by crisis elsewhere.

With control over Tanupar, the Aspawatam campaign was  able to overwhelm the chiefdoms of the Nation, who were too spread out and too fractious to mount a cohesive defense. The last chiefs understood that they had no hope of winning and retreated to the south and swore fealty to the Tuaxi and Malkh kings.

The difficult terrain and superior fortifications of the southern cities and towns halted the Aspawatam advance. The chariots favored by the Aspawatam are of limited use in the rough terrain, forests, and mountains of the south, where Tuaxin mounted infantry is entirely dominant.