The Mongol invasion of China lasted over 6 decades and particularly involved the defeat of Jin Dynasty, Western Xia, and theSouthern Song, which finally fell in the year 1279; the year that the Mongols had claimed the total conquest of China.
At the time of the kurultai, Genghis was involved in a dispute with Western Xia — which eventually became the first of his wars of conquest. Despite problems in taking well-defended Western Xia cities, he substantially reduced the Western Xia dominion by 1209, when peace with Western Xia was made. He was acknowledged by their emperor as overlord. This marks the first in a line of successes in defeating all the kingdoms and dynasties in China which wasn't complete until Kublai Khan's rule. A major goal of Genghis was the conquest of the Jin Dynasty with the aid of the Song Dynasty, allowing the Mongols to avenge earlier defeats, gain the riches of northern China and mostly to establish the Mongols as a major power among the Chinese world order.
He declared war in 1211, and at first the pattern of operations against the Jin Dynasty was the same as it had been against Western Xia. The Mongols were victorious in the field, but they were frustrated in their efforts to take major cities. In his typically logical and determined fashion, Genghis and his highly developed staff studied the problems of the assault of fortifications. With the help of Chinese engineers, they gradually developed the techniques to take down fortifications. Islamic engineers joined later and especially contributed counterweight trebuchets, "Muslim phao", which had a maximum range of 300 metres compared to 150 metres of the ancient Chinese predecessor. It played a significant role in taking the Chinese strongholds and was as well used against infantry units on battlefield. This eventually would make troops under the Mongols some of the most accomplished and most successful besiegers in the history of warfare.
As a result of a number of overwhelming victories in the field and a few successes in the capture of fortifications deep within China, Genghis had conquered and consolidated Jin territory as far south as the Great Wall by 1213. He then advanced with three armies into the heart of Jin territory, between the Great Wall and the Yellow River. With the help of Chenyu Liu, one of the top officers who betrayed Jin, Genghis defeated the Jin forces, devastated northern China, captured numerous cities, and in 1215 besieged, captured and sacked the Jin capital of Yanjing (the modern-day Beijing). But the Jin emperor, Xuan Zong, did not surrender, but moved his capital to Kaifeng. There his successors were eventually defeated in 1234.
The vassal emperor of Western Xia had refused to take part in the war against the peoples of the Khwarizm, and Genghis had vowed punishment. While he was in Central Asia, Western Xia and Jin had formed an alliance against the Mongols. After rest and a reorganization of his armies, Genghis prepared for war against his biggest foes. By this time, advancing years had led Genghis to prepare for the future and to assure an orderly succession among his descendants. He selected his third son Ogedei as his successor and established the method of selection of subsequent khans, specifying that they should come from his directdescendants. Meanwhile, he studied intelligence reports from Western Xia and Jin and readied a force of 180,000 troops for a new campaign.
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