The Shattered Empire (Swords & Wizardry)

From publisher blurb:

For millennia the men of the north were pitted against one another in ceaseless battle, one city-state against another. In due time, the men of Yal-Garok gained the upper hand, dominating the minor city-states through arranged marriages and then striking at its greatest rivals militarily and forcing them to sue for peace. The rise of Yal-Garok ushered in the Age of Peace, allowing the clever, hard-working Northmen to focus their energies are building rather than destroying one another. Most of the country was brought under cultivation, towns and cities flourished and colonies were established in the eastern mountains, the western prairie and, unsuccessfully, the icy woodlands of the far north.