Roughly five hundred years ago, The Elven Kingdom annexed the Harvest Hills Region in a short but bloody conquest. This colonial expansion was supposed to be a first step towards conquering Iron Maw Mountains, but it stalled due partially to a more pressing war with Gaultonnia, and the fervent defense of the region by the Hills Rising Movement, who were at the time supplied and supported by different Enclaves from the Iron Maw Mountains. Consolidation instead of expansion was also moved to the front when the 11th Foot and Arrow Battalion was lost in the Lightning Swamp, in an effort to crush the Hills Rising Movement.
So instead of pushing forward, the Elven Kingdom consolidated their gains, and since then the Harvest Hills Region has been part of the Elven Kingdom, to various degrees. Currently the Elven Kingdom has the least control that they've ever had due to the fall out of The Night War.