context: The Mongols learned from captives of the abundant green pastures beyond the Bulgar territory, allowing for the planning for conquest of Hungary and Europe. Genghis Khan recalled Subutai back to Mongolia soon afterwards, and Jebe died on the road back to Samarkand. The famous cavalry expedition led by Subutai and Jebe, in which they encircled the entire Caspian Sea defeating all armies in their path, remains unparalleled to this day, and word of the Mongol triumphs began to trickle to other nations, particularly Europe. These two campaigns are generally regarded as reconnaissance campaigns that tried to get the feel of the political and cultural elements of the regions. In 1225 both divisions returned to Mongolia. These invasions added Transoxiana and Persia to an already formidable empire while destroying any resistance along the way. Later under Genghis Khan's grandson Batu and the Golden Horde, the Mongols returned to conquer Volga Bulgaria and Kievan Rus' in 1237, concluding the campaign in 1240.
question: the leader of the Mongols was who
answers.text: Genghis Khan