From the Steppe to the Civilizational Gamble of Eurasia

Mongolia gives people the impression that it is poor, backward, sparsely populated, and loose. However, under the leadership of Temujin, a child whose father was killed at a young age, Mongolia unified step by step, established a highly militarized army, and established an unprecedented territory across the Eurasian continent. How did he do it? How did his descendants inherit and continue to expand? Why was it finally annihilated?