I recently read the Wikipedia article on Julius Caesar's Gallic wars, and I had trouble understanding his motives for attacking the Helvetti. It seemed to me that they were simply looking to migrate west and had even secured permission from neighboring Gallic tribes to do so. Can someone explain the background to this historical event?
Well, that wasn't a peaceful mass migration (if there was even such a thing). The Helvetii moved west, if not with conquest of Gaul in aim, then with intent to displace the locals wherever they were going to settle down. That would set off a chain of events that disturbs the balance already existed. Rome also concerned that a German tribe would move into the space vacated by the Helvetii.
Caesar's first book of "Commentarii de bello Gallico" dealt specifically with your question.