Final answer:
The direct cause for the formation of the Mongol Empire was the unification of Mongol tribes by Temujin, who is also known as Genghis Khan.
Step-by-step explanation:
The formation of the Mongol Empire was led directly by the unification of Mongol tribes by Temujin, who later became known as Genghis Khan. This unification was a remarkable feat of organization and leadership, laying the foundation for one of the largest empires in history. Under Genghis Khan's rule, the Mongols implemented strategic military tactics, including the use of siege technology like catapults, and developed trade infrastructure which played crucial roles in their empire's expansion and consolidation.