Reports of the Mongol attacks terrified Europe. The Mongols increased their empire using swift and decisive attacks with an armed and disciplined cavalry. They wiped out the populations of some entire towns that resisted—as was their usual policy—depopulating some regions and confiscating the crops and livestock from others. This type of total warfare spread panic even among Europeans not directly affected by the Mongol onslaught and sent refugees fleeing westward.
If your talking about Alexander the great I think it was because his army begged him to turn around and go back home because of the harsh weathers and they were very tired.