Empires have been built throughout history. Apart from those that are built on the basis of religion, what would be the real reasons for their expansion?
1.Would it more of material greed or more of noble visions of the leaders? For example to achieve unity amongst nations, to provide prosperity for humanity, etc?
2.To enhance the expansion, the conquerers have to enlist the conquered people as part of their army. How would they be sure of the loyalty of the soldiers?
3.In the particular case of the ransack of Baghdad, the Mongols basically destroyed the city and its people. What much benefits would be a destroyed city to the Mongols? Also the actions would bring much hatred and enmity amongst the survivors. Would it not be much better to only attack the vital parts of the city?
There are numerous reasons that underlie the quest for empire and expansion. These include fear of the aggression of others, thus the need to establish buffer states for protection; glory for a nation's leaders; acquisition of land for farming or as gifts to conquering generals; natural resources such as gold, silver, etc.; strategic location of area to be conquered (trade); economic investment in new areas.
In the case of the Mongols, it is more important to look at how they subdued conquered territories as opposed to what they conquered. Genghis Khan and his successors deliberately used terror as a weapon of war. If a city he was besieging gave up without a fight, its people would usually be spared but would have to submit to Mongol control. If the city fought against the Mongols, everyone, including civilians, would be massacred. This reign of terror is a large part of why they were so successful. People were more willing to give up than to suffer massacres at his hand. For example, when he besieged the city of Herat, in present-day Afghanistan, he killed over 1,600,000 people. (lostislamhistory.com)