Complex sentence: Though the Alps form a barrier they have never formed an impassable barrier, since, from the earliest days onwards, they have been traversed first, perhaps, for purposes of war or commerce, and later by pilgrims, students and tourists.
Split into simpler sentences: Though the Alps form a barrier they have never been an impassable barrier. From earliest days onwards, they have been traversed first, perhaps, for purposes of war or commerce, & later by pilgrims, students and tourists.