Titus Livius (Livy), The History of Rome, Book 34
Evan T. Sage, Ph.D. Professor of Latin and Head of the Department of Classics in the University of Pittsburgh, Ed.

28. Quinctius had now made adequate preparations and had left his base and on the second day arrived at Sellasia above the Oenus river, at which place it was said that Antigonus, king of the Macedonians, had contended in pitched battle with Cleomenes, the tyrant of the Lacedaemonians. [2] Having learned that the descent was through a difficult and narrow pass, he sent men ahead to build a road over a short bypass over the mountains, and came by a sufficiently wide and open way to the river Eurotas, which flows almost under the very walls of the city. [3] When the Romans were laying out their camp and Quinctius himself with the cavalry and light infantry had gone on ahead, the auxiliary forces of the tyrant attacked and threw them into panic and disorder, since they anticipated no such event, because no enemy had shown himself on their entire march and they had passed through an apparently peaceful country. [4] For a time there was confusion as the cavalry called to the infantry and the infantry [p. 489]to the cavalry, since each lacked confidence in itself; —¹ [5] finally, the standards of the legions came up, and when the leading cohorts of the column were thrown into the fight, those who had a moment before been a cause of terror were driven in panic into the city. [6] The Romans retired so far from the wall that they were beyond spear-range, and forming their line waited for a while; when no enemy came out against them, they retired to the camp. [7] The next day Quinctius began to lead his troops in array along the river and past the city to the foot of Mount Menelaus; at the head marched the legionary cohorts and the light infantry and the cavalry formed the rear-guard. [8] Inside the wall Nabis had formed and made ready his mercenaries, in whom he placed most confidence, to attack the enemy in the rear. [9] When the tail of the column had passed, they burst from the town through several gates at once, with the same fury as the day before. [10] Appius Claudius was bringing up the rear; he had prepared the minds of his men for what was likely to occur, lest it catch them unawares, and he at once faced about and presented a solid front to the enemy. [11] So, just as if organized² battle-lines had met, there was a regular battle for a while; at length the soldiers of Nabis turned in flight; and this would have been less hazardous and dangerous if the Achaeans, who knew the ground, had not pressed them hard. They caused great slaughter and disarmed many of them as they scattered in flight in every direction. [12] Quinctius encamped near Amyclae, and from that base he laid waste all the well-populated and pleasant [p. 491]country districts which surrounded the city, and³ when no enemy now left the gates he moved his camp to the river Eurotas. Next he devastated the valley that lies below Taygetus and the fields that stretched towards the sea.