Making war in winter is tough. To add to the usual logistical challenges of supplying an army, i.e., sufficient food, water, arms and ammunition, food/water for any beasts of burden or cavalry horses, etc. etc. you now have to add the problem of keeping everyone sufficiently warm, traveling through snow or ice if the region is one with ground accumulation, getting sufficient (dry) timber for the "keeping-everyone-warm" problem, ....