de bello gallico, 4.30

With these things having been known, the chiefs of Britain, who after the battle had convened to Caesar, having spoken amongst themselves, because they understood that cavalry and ships and grain were lacking for the Romans and they learned the paucity of the soldiers from the smallness of the camp, which were more narrow by this (fact) that Caesar had transported his legions without baggage, they concluded it was the best thing to do, with the rebellion having been made, to prohibit our men from grain and provision and stretch the affair into winter, because, with these men having been conquered or kept from return, they were believing that no one would cross over into Britain afterward for the sake of waging war. And so, with a conspiracy again having been made, they began little by little to depart from camp and led their own men secretly from the fields.