How did the defeat of the Naiman tribe open Genghis Khan’s path westward?

The defeat of the Naiman tribe was a pivotal moment in Genghis Khan’s expansion, as they were the last of the powerful tribes in Mongolia proper. Following their defeat, the son of the Naiman ruler, Küchlüg, fled to the kingdom of Karakhitai, where he eventually usurped power.12

By subsequently taking control of the lands of the Karakhitai, Genghis Khan effectively opened the path westward toward Khwārezm, a region featuring oases along the Amu Darya. This westward movement provided Genghis Khan with a strategic alternative to his campaigns in China and secured his rear and flanks against the potential threat of other nomadic powers organizing military forces backed by urban and agricultural resources.12