The keys of the Mongols’ successes were mobility and discipline. Genghis Khan created an army of incredible organization and discipline, unlike any before seen in Mongolia. Composed primarily of cavalry archers, they could move with amazing speed and essentially outmaneuver any opponent.
Genghis Khan also recognized the importance of military intelligence. He crafted a powerful network of spies so that he would always know the movements of his enemies. While no one could ever be completely sure of exactly where Genghis Khan and his armies were, he always knew exactly where everyone else was.
Genghis Khan also realized that while his armies were unbeatable on the battlefield, cavalry would not be able to take a walled city. He developed his own technology of siege warfare, creating catapults and other siege machines that could be moved easily and built rapidly on the field of battle. This would assist him greatly in his conquest of China and the rest of Asia.
China
The Mongols had always raided the Chinese from time to time, but under Genghis Khan they would not only raid but also conquer. It was only natural that the Chines, enemies of the Mongols for so long would be the first part of their world conquest.
Starting in 1206 the Mongols invaded northern China, ruled by the Tangut. They were north of the Great Wall, and the province was made up mostly of farmers and herders. Genghis fought their armies for four years, finally in 1210 being declared the ruler of the province.
After conquering the province, the Chin in Beijing sent a delegation to Genghis Khan, declaring that he must submit to them as their vassal and pay tribute. Rather than submit, Genghis Khan and his armies invaded. For six years they fought, until the Chin finally gave in to Genghis Khan and submitted to him, paying tribute. Northern China completely belonged to the Mongols.