true, you might want to look at empires that lasts for centuries after rapid expansion

alexander the great, napoleon, the japanese (1940ish) don't count because they cannot sustain the expansion

ironically, mongol empire (and its vast expansion) lasted for 3.5 centuries. there must be some other reasons that neither cruelty nor barbarism can explain (in fact, empires based on cruelty/barbarism don't last long)