What factor(s) contributed to the Mongols conquering most of the known world?

The decline of rome was the natural and inevitable effect of immoderate greatness. prosperity led to decay, the causes of destruction multiplied with the extent of conquest; we should be more surprised the empire did not collapse sooner. -- edward gibbon, historian and politician, 1776-1788 ? you! which of the following causes does gibbon highlight (check all that apply)? 1.) wealth led to a decay of roman values. 2.) the empire was too large to govern. 3.) labor-intensive agricultural methods led to a reliance on slave labor. 4.) the lack of economic freedom discouraged hard work innovation. 5.) the common citizens did not have any say in government and no way to seek reform.