The Mongol invasion of Khwarezmia lasted from 1219 to 1221. It marked the beginning of the Mongol Conquest of the Islamic States, and it also expanded the Mongol invasions, which would ultimately culminate in the conquest of virtually the entire known world, save for Western Europe, Scandinavia, Arabia, Africa, South Asia, Japan and parts of Southeast Asia.
Ironically, it was not originally the intention of the Mongol Khanate to invade the Khwarezmid Empire. Indeed, Genghis Khan had originally sent the ruler of the Khwarezmid Empire, Ala ad-Din Muhammad , a message greeting him as his equal: "you rule the rising sun and I the setting sun." The Mongols' original unification of all "people in felt tents", unifying the nomadic tribes in Mongolia and then the Turcomens and other nomadic peoples, had come with relatively little bloodshed, and almost no material loss. Even his invasions of China, to that point, had involved no more bloodshed than previous nomadic invasions had caused.
It would be the invasion and utter destruction and complete devastation of the Khwarezmid Empire which would earn - and rightly - the Mongols the name for bloodthirsty ferocity that would mark the remainder of their campaigns. In this brief war, lasting less than two years, not only was a huge empire destroyed utterly, but Genghis Khan introduced the world to tactics that would not be seen again until the Germans used them so well in World War II - indirect attack, and complete and utter terror and slaughter of populations wholesale as weapons of war.