Final answer:
Genghis Khan united the Mongol confederation into an empire by implementing a legal and institutional framework, decimalizing the army, and launching military conquests.
Step-by-step explanation:
Genghis Khan united the Mongol confederation into an empire through various strategies and reforms.
Firstly, he implemented a legal and institutional framework to eradicate divisions between tribes and promote merit-based advancement.
Additionally, he decimalized the army and organized soldiers into groups of ten known as arban, which fostered unity and loyalty.
Furthermore, Genghis Khan initiated military conquests, initially focusing on looting resources but later expanding territorial control.