The Gallic War by Julius Caesar - Book 1, pt. 5
After his death, the Helvetii nevertheless attempt to do that which they had resolved on, namely, to go forth from their territories. When they thought that they were at length prepared for this undertaking, they set fire to all their towns, in number about twelve-to their villages about four hundred-and to the private dwellings that remained; they burn up all the corn, except what they intend to carry with them; that after destroying the hope of a return home, they might be the more ready for undergoing all dangers.
They order every one to carry forth from home for himself provisions for three months, ready ground. They persuade the Rauraci, and the Tulingi, and the Latobrigi, their neighbors, to adopt the same plan, and after burning down their towns and villages, to set out with them: and they admit to their party and unite to themselves as confederates the Boii, who had dwelt on the other side of the Rhine, and had crossed over into the Norican territory, and assaulted Noreia.
Source: Wikisource Translation based on W. A. McDevitte and W. S. Bohn (1869)