Titus Livius (Livy), The History of Rome, Book 38
Evan T. Sage, Ph.D., Ed.

40. Having published these treaties and decrees, Manlius with the ten commissioners and all the army set out for the Hellespont, summoning thither the chiefs of the Gauls, and stated the terms on which they should observe [2??] peace with Eumenes,¹ and warned them that they should discontinue their habit of wandering about under arms and should keep themselves within the boundaries of their own lands. [3] Then, collecting ships from the whole coast, the fleet of Eumenes also being brought up from Elaea by his brother Athenaeus, he ferried all his forces across' to Europe.² [4] Then, leading a column heavily laden with every sort of booty by short stages through the Chersonese he established a base at Lysimachia, in order that with his pack-animals as far as possible fresh and in good condition he might enter Thrace, the journey through which was generally feared. [5] On the day of his departure from Lysimachia he reached the river called Melas and on the day after that Cypsela. [6] From Cypsela [p. 137]a road awaited him, for about ten miles wooded,³ narrow, rough, and by reason of the difficulty of this route he divided the army into two sections, and, having ordered one to go ahead and the other to bring up the rear at a great distance, he placed the baggage between them; there were carts loaded with public money and other valuable booty. [7] As they were marching in this order through the defile, not more than ten thousand Thracians, of four tribes, the Astii and the Caeni and the Maduateni and the Coreli, blocked the road at the narrow point.⁴ [8] It was generally believed that this did not happen without treachery on the part of Philip, king of the Macedonians; he knew that the Romans would return by no other route than through Thrace, and he knew how much money they would bring with them. The commander was with the van, being concerned about the unfavourable character of the terrain. [9] The Thracians did not move until the armed troops were past; when they saw that the van was out of the defile and that the rear was [10??] not yet at hand, they fell upon the trains and the baggage, and having killed the guards some of them carried off what was in the wagons, others drove off the pack-animals, loads and all. [11] When the uproar reached first those who were following and just entering the defile and then came to the head of the column, there was a rush from both directions towards the centre, and a disorderly battle began in several places at once. [12] The Thracians were exposed to slaughter, hampered as they were by the burden of the booty itself, while many of them were without arms, in order that they might keep their hands free to plunder; the Romans were betrayed by the unfavourable ground, since the barbarians [p. 139]charged them over familiar paths and sometimes laid⁵ ambushes for them in low-lying valleys. Even the loads and the wagons, inconveniently placed for one side or the other, as chance determined, hindered the fighters. [13] Here the plunderer fell, there the defender of the plunder. [14] Just as the terrain was unfavourable or favourable for one party or the other, just as the spirits of the fighters varied, just as their numbers —for some met parties larger than their own and others smaller —just so the fortune of the battle changed; many fell on both sides. [15] Night was now at hand when the Thracians retired from the fight, not to avoid wounds or death, but because they had enough of spoils.⁶