I don't understand the lull. Isn't this the perfect time to really hurt your enemies?
On one hand, I can see how it might be ideal to march abroad during winter but at the same time, with the new provisions requirement, field stays have shortened considerably. I was used to staying in the field for a month at a time, now I'm lucky to get a week or two.
At the same time, I'll use RL logic regarding winter: who wants to march in snow, or fight in the freezing wind? Winter wars make people absolutely miserable on each side. Invasions of western Russia by any number of militaries springs to mind...