Why did Genghis Khan launch his final campaign against Xi Xia?

Genghis Khan launched his final campaign against the Xi Xia (also known as the Tangut kingdom) in 1226–1227 primarily because the Xi Xia king had refused to provide military assistance to the Mongols during their expedition against Khwārezm.1

Prior to this, the relationship between the two powers had been strained; while the Xi Xia had previously declared allegiance to the Mongols, their failure to support the Khwārezm campaign prompted a punitive response. Genghis Khan viewed the wealthy Tangut state as a necessary objective to secure before committing his main forces to the full-scale conquest of China. It was during this successful, albeit destructive, campaign that Genghis Khan died in 1227.123