[Part 1 of 2]
They weren't inevitable. In particular, the Mongol conquest of Central Asia, a prerequisite for their invasions of Western Asia and Europe, doesn't look like it was inevitable.
Genghis Khan completed his unification of Mongolia in 1206, with his final defeat of his rival Jamukha (previously his ally and friend), who had been elected Gür Khan ("universal ruler") 5 years earlier. Later in 1206, Genghis Khan (at the time, Temijin) was elected Genghis Khan, and was the supreme ruler of the Mongol Steppe (and perhaps, depending on the exact meaning of the title, in theory the supreme ruler of the whole world). This young (and still small) Mongol Empire was at least somewhat expansionist, proceeding to conquer the forest peoples to the north of the steppe. While a small part of the Mongol army was doing this, the majority of the army moved against Xi Xia.
At the time, Mongolia and northern China has seen about two centuries of three-way warfare for the control of the region. Originally, the three main participants had been Song China, the Liao (the Khitans) based in Manchuria, and Xi Xia (the Tanguts) in what is now western Inner Mongolia and nearby areas, as shown in this map. The Liao/Khitans were defeated by the Jin (the Jurchens), also from Manchuria. Substantial parts of the Liao/Khitans fled west, and established a new state in Central Asia, extending into Transoxiana. This new state was Qara Khitai AKA Western Liao (light green in this map which also shows Xi Xia (turquoise), the Jin Empire (grey) and Song China (gold)). A catastrophic defeat of the Song by the Jin led to the Song losing most of their territory north of the Yangtze, and changed the relationship between Xi Xia and Jin from alliance against the Song to being adversaries.
Xi Xia had a long history of involvement in steppe politics, and had given refuge to Kerait refugees after their defeat by the Mongols. This led to Mongol raids on Xi Xia, and likely contributed Xi Xia being a major early target of the Mongol Empire. From 1207 to 1210, the Mongols launched further major raids, and then a longer-lasting major invasion of Xi Xia, leading to Xi Xia submitting to the Mongols as a vassal.
Still in 1210, Xi Xia proceeded to fight the Jin, on the basis that the Jin had refused to help them against the Mongols. At the same time, the Jin demanded the submission of the Mongols to Jin overlordship. The Jin, controlling northern China, were formidable, and the Mongols took some time to prepare for war. In 1211, the Mongols joined the war against the Jin alongside Xi Xia. The war against the Jin would last until 1234, after the death of Genghis Khan, and was followed almost immediately by war against Song China. The Song had provided very valuable logistic aid for the Mongols, helping them finally defeat the Jin, and then proceeded to invade Mongol-held China in order to get a bigger share of the former Jin territory (which was former Song territory). After some years of war with the Mongols, they had cause to regret that decision, but their resistance against the Mongols lasted against 1279, during the rule of Kublai Khan, the 4th successor to Genghis as leader of the Mongol Empire.
This 70+ years of warfare in the east could easily have kept Mongol attention away from Central Asia until civil war led to Mongol power decaying enough to prevent their conquest of Central Asia.
What remains is to look at why the Mongols went west, despite the wars in the east. The first major step for the Mongols to the west was the conquest of Qara Khitai. First, just as Xi Xia had given refuge to defeated Keraits, Qara Khitai took in refugees from the Mongol defeat of the Naimans. These Naiman refugees were led by Kuchlug (one of the two main Bad Boys of the Secret History of the Mongols, the other being Jamukha). Kuchlug became the son-in-law of the Gur Khan of Qara Khitai, and also formed an alliance with Muhammad II, shah of the Khwarazmian Empire, which was at war with Qara Khitai for the control of Transoxiana (this kind of treachery is why Kuchlug was famed as a Bad Boy). With the Gur Khan distracted by the war with Khwarazm, and by local revolts, Kuchlug revolted, aiming for control of Qara Khitai. The revolt did not go well for Kuchlug at first, but the next year he managed to ambush and capture the Gur Khan, and "encouraged" him to abdicate. This abdication led to Kuchlug becoming the new Gur Khan, at the cost of the loss of Transoxiana to Muhammad. During this, the Karluks (a Turkic people), subjects of Qara Khitai, revolted (successfully), and became vassals of the Mongol Empire. In 1216(?), Kuchlug moved to reconquer the Karluks, who appealed to Genghis Khan for aid. With one of their vassals attacked by one of their old Naiman enemies, the Mongols moved west. At war with the Jin, Genghis Khan sent a relatively small Mongol army, together with Uighur troops (who were Mongol vassals) against Kuchlug, and defeated him, greatly helped by Kuchlug's unpopularity among the population of Qara Khitai. This left the Mongols as the rulers of Qara Khitai - and also a border with Transoxiana, now under Khwarazmian rule.
The next step was war with, and the conquest of, the Khwarazmian Empire. Genghis Khan's plan appears to have been to continue fighting the Jin, while maintaining peace with the Khwarazmian Empire, and improving trade relations with the Khwarazmian Empire (the long war between Khwarazm and Qara Khitai had probably hurt trade). His proposal to the shah of Khwarazm can be paraphrased as "I rule the east; you rule the west. Let us conclude a firm agreement of friendship and peace." However, it took little time for the Khwarazmians to provoke war: the governor of Otrar (who was an uncle of Muhammad II) arrested a Mongol trade caravan, executing all of the people (about 500) and confiscating the trade goods. Mongol envoys to the shah, protesting this and demanding compensation and the handover of the guilty governor, were executed by the shah. This was not behaviour that met Mongol approval. The Mongol response was much more severe than their response to Qara Khitai's attack on their vassals the Karluks: Genghis Khan assembled a large army, and led it in person (probably the largest army that Genghis Khan ever led). Muhammad-shah adopted a relatively passive defensive strategy, depending on his strongly fortified cities to stop the Mongols (probably because his mobile field army was much weaker than the Mongol army). It didn't work, in part because loyalty to Muhammad was minimal. The area had only recently been conquered, and the decades of fighting between Khwarazm and Qara Khitai had taught local rulers that defecting to a powerful invader was often a successful strategy to protect their cities (they had also learned that they could usually successfully revolt against the invader once their armies had left the area, which turned out to work very poorly against the Mongols who tended to return and destroy such revolting cities). Also, the Mongols turned out to be adept at attacking fortified cities - it has been suggested (plausibly) that this was due to their adoption of Chinese siege technology (and the inclusion of Chinese siege engineers in their army), including gunpowder weapons.
[continued in the next post]
