Today we take it for granted that waging an aggressive war is immoral. When a nation goes to war it seeks to justify the decision to their own citizens and to the world at large. Usually it tries to cast its aggression as self-defense, or as necessary to right a historic wrong, or as being good for the inhabitants of the invaded lands. Sometimes false flag operations are staged as a pretext for fighting. Everyone from the Spanish Conquistadors to the American Revolutionaries to the Nazis to today's Russians has argued for the morality or necessity of their war-making.
My question is, did the Mongol conquerors also attempt to moralize their conquests? Or did they take it for granted that the strong could take what they wanted from the weak?
They did.
For the initial conquest of Mongolia, the opponents of the Mongols were either traditional enemies, or there was a recent cause (e.g., a recent attack on the Mongols or their allies). Alliances between tribes made enemies as allies would join together in an attack on the Mongols, making a tribes joining such an alliance enemies of the Mongols. An attack on an ally of the Mongols would make a tribe an enemy of the Mongols. This web of enemies and allies even brought war between the Mongols and the major sedentary neighbours (the Jin, Xi Xia, Qara Khitai). For example, the Tatars had poisoned Temujin's (i.e., Genghis Khan, to use his later title) father, and the Merkits had raided the Mongols and stolen Temujin's wife. The Naimans had supported a coup which overthrew Toghrul (AKA Ong Khan AKA Wang Khan), the khan of the Keraits, who was an ally of Temujin. The Jin (AKA the Jin Dynasty, ruling northern China, AKA Jurchens) were traditional supporters of the Tatars. After relations between Toghrul and Temijin broke down, they went to war with each other. When Toghrul was very old, his son became the effective leader of the Keraits, and restarted the war against the Mongols. After defeat, he took refuge in Xi Xia (AKA Western Xia, AKA the Tangut Empire), which led to Mongol raids on the Tanguts, and the submission of Xi Xia to the Mongols as a vassal. While Qara Khitai (AKA the Western Liao) was involved in war with its western neighbour, the Khwarezmian Empire, and also civil war, some of their eastern-most subjects became Mongol subjects. After the civil war in Qara Khitai ended, they went to war to reconquer those former subjects, leading to war with the Mongols (and the conquest of Qara Khitai).
By this time, Mongolia was united under Genghis Khan, but the war against the Jin was still continuing, with no end yet in sight. The Mongols don't appear to have wanted to add to their wars at this point. The Mongol conquest of Qara Khitai had made them neighbours of the Khwarezmians, and they sought peaceful trade. However, the Khwarezmian governor of a border region (and close relative of the Khwarezmian shah) massacred a Mongol trading caravan, and the shah refused to hand over the governor to the Mongols. The Mongols went to war with, and conquered, the Khwarezmian Empire.
Next, the Mongols turned on Xi Xia. Xi Xia was a vassal of the Mongols, but refused to provide troops for the war against the Khwarezmian Empire. Genghis Khan attacked his disloyal vassal (dying mid-conquest), and Xi Xia was destroyed as a state.
After the death of Genghis Khan, a new ideological justification for conquest became important: Heaven had chosen Genghis Khan, the Son of Heaven, to rule, and his descendants, the Golden Clan, after him. The Great Mongol Nation thus had the Heaven-given overlordship over the entire world. Mongol diplomacy came to begin with a declaration of this heavenly mandate, and often included a demand for submission as a vassal. For example, a Mongol message to Japan finished with
But now, under our sage emperor, all under the light of the sun and the moon are his subjects. You, stupid little barbarians. Do you dare to defy us by not submitting?
and a message to the French included
We, with the force of Eternal Heaven, Hulegu, eager to destroy the perfidious nations of the Saracens, to the illustrious King of France Louis. We inform you that you must obey us without hesitation as we invoke the name of the living God; he has granted us power
This style of diplomacy automatically gave the Mongols a just cause for war (in their own eyes) if the recipient of the demand refused to submit and accept their rightful position as a vassal. There were, in this political philosophy, only two types of non-Mongol nations: those that were loyal Mongol vassals, and those in revolt against Heaven-granted Mongol rule. Refusal to accept Mongol rule wasn't just defiance of the Mongols - it was also defiance of the will of Heaven.
This justification could easily be combined with others. For example, the conquest of Song China was justified by Song refusal to submit as a vassal, and also by Song attacks on the Mongols (the Song had celebrated the victory of the Mongol-Song alliance over the Jin by invading Mongol-held territories to try to grab more than the alliance agreement had promised). Similarly, if a local ruler was offended by the demand to submit and executed the Mongol ambassadors, the Mongols would justify war on the grounds of the execution of their ambassadors.