The reports as to the passes of the Alps were less satisfactory.

Those who had examined them found that the difficulties they offered to the passage of an army were enormous, and that the tribes who inhabited the lower passes, having suffered in no way yet at the hands of Rome, would probably resist any army endeavouring to cross.
By far the easiest route would be to follow the seashore, but this was barred against the Carthaginians by the fact that the Massilians (the people of Marseilles) were the close allies of Rome.

They had admitted Roman colonists among them, and carried on an extensive trade with the capital.

Their town was strong, and their ports would be open to the Roman fleets.

The tribes in their neighourhood were all closely allied with them.
Hannibal saw at once that he could not advance by the route by the sea without first reducing Marseilles.