From the Huns through the Mongolians, why were nomadic people from the east able to create such havoc in Europe?  Was it an endless 'first mover' advantage with horse domestication?  Was it a higher protein diet relative to the more farm-centric Europeans?  Why weren't Celts, Ostrogoths, Teutonic knights, or Slavs pushing east?  Were Europeans just wasting all their expansive energy on the Crusades?