So I just got back from my first trip to Europe, and flew over France and the Swiss Alps into Milan.
Flying over that massive, beautiful mountain range, I couldn't help but wonder - how did the Romans, in 50 BC, get their armies and supplies to France in a consistent and stable way in order to support their campaigns in western Europe? Did they simply use ships and sail around them, or did they do like Hannibal did later and go through mountain passes?
I've searched a bunch, and while I can find a lot of information about the political situation, I can't find much about the logistics of the actual battles, or troop transport.
50 BC is a different story, but in the Second Punic War, the Romans sent their army to Marseille on ships in an attempt to cut Hannibal off from Italy (they were late). The Northern reaches of Italy, Liguria or Cisalpine Gaul, took the Romans a long time to conquer. At least one commander destined for Spain was killed on his way through Liguria in the second century (Livy 37.57). However, those regions were completely integrated after the Social War. This meant that the march through the Alps was much safer than it had been in the time of Hannibal.