I want to point out that the reason the mongols took over so much land and actually kept it was because they did NOT annex into their 'country'. They gave them a choice between a) joining our empire, stay in charge, pay us a cut of taxes, listen when (if) we make demands...or b) we will kill the current rulers who didn't choose a, put others in charge and give them the choice again.
Needless to say, people chose option a, leading to the 'omfgtwfbbq' size of the mongol empire and some silly easy conquests when the people in charge had the people not in charge realize that they could keep themselves alive by NOT fighting.
Note, this is what I was taught over twenty or so years ago in school, god knows what they say now >.>
Yes, there was looting, pillaging, and all those hijinks, but honestly, everyone in europe was busy doing that to themselves anyways so it wasn't really anything new...