The Alps are a very high mountain range, which is also very dangerous in every time of the year, but especially during the winter. In the antiquity, without having any modern day equipment, Hannibal tried to cross this mountain range in order to enter the Roman Empire and attack Rome.
There were lot of problems that Hannibal and his troops faced because of this move. The temperatures were very low, and lot of people suffered and died from hypothermia, as well as the war elephants that were not able to sustain the freezing temperatures. The Alps do not offer proper shelter, especially not for large mass of people, which contributed to the previously mentions hypothermia. Also, a lack of food was a huge problem, as in winter there's pretty much nothing on the Alps that can provide food, apart from hunting wild animals, but there's not dense populations of them, so the malnutrition caused weakening of the people as well as being more prone to sickness.
All in all, Hannibal's army was decimated after the crossing of the Alps.
