What are the reasons for the belligerence of the state of Genghis Khan?

Genghis Khan in 1221 set the task for his sons to go through eleven countries and peoples and reach the Kivamen-Kermen – Kiev. The main task of the mighty ruler was the conquest of the Kipchaks, but in 1223 they met with resistance from the princes of Rus. Having defeated them on Kalka, in the future, the Mongols abandoned the campaign. They suffered a series of defeats, and went deep into their territories. The Mongol army was reformed by Genghis Khan after the unification of the tribes, and was a formidable force thanks to the tactics of raids by swift cavalry units of light archers and cavalry.