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?
Empires are generally not brought about by religious expansionism but they arise as a natural out growth of a progression of cultural and social factors. As a city grows in size the development of a strong centralized government grows with it. Cities with common language religious beliefs stories and cultural origin begin to band together for mutual benefit and some times protection from the "outside". Typically the strongest of the cities takes a leadership role. This is typically how most empires start. Then when outsiders encroach or attack, the loose confederation of cities band together in defense and if they win the fight get the spoiles of war and the now defeated territory. Thus an empire is born.
Some leaders were benign to those conquered while others demonstrated uncommon cruelty. Those the treated the new subjects with respect and equality generally benefited by new found loyalty and trustworthy additions to the army. Others could not count on loyalty unless subjugated in some way.
Leaders like Alexander the Great allowed conquered peoples to govern themselves, and thus won their loyalty. Gengis Khan on the other demanded loyalty and left behind sons as governors of conquered territories. Those cities or minor "empires" that refused or showed any disloyalty was destroyed. Khan was ruthless but an extraordinary leader and man of vision, as was Alexander.
I hope this answers your questions, if not please ask additional questions..