Final answer:
The Hunnu's recruitment of Yenisei Kyrgyz soldiers was primarily to strengthen their military power, which played a crucial role in maintaining their dominance and exerting influence in the region. The correct answer is option 1.
Step-by-step explanation:
The recruitment of soldiers by the Hunnu (also known as the Xiongnu) from the Yenisei Kyrgyz in the 1st century BC was primarily undertaken to strengthen their military power. The Xiongnu were a powerful confederation known for their military might and used various strategies to maintain and expand their dominance. Their relationship with adjacent civilizations, like the Han Chinese, was complex, involving periods of trade, warfare, and tribute exchange. The Xiongnu sought to consolidate their military capabilities by integrating soldiers from various surrounding peoples, like the Yenisei Kyrgyz, and employing them in both offensive expansions and defensive maneuvers in their extensive territorial control. Recruiting soldiers from the Kyrgyz and capturing other groups provided the Xiongnu with necessary manpower to continue exerting pressure on the Chinese Empire and other entities, thus maintaining their formidable presence in the region.