Thucydides, History of the Peloponnesian War
Benjamin Jowett, Ed.

64. Meanwhile word was brought from their friends in Tegea that they must come at once, since Tegea was about to secede and had almost seceded already to the Argives and their allies. [2] Whereupon the Lacedaemonians led out their whole force, including the Helots, with an alacrity which they had never before¹ displayed, and marched to Orestheum in Maenalia. [3] They told their Arcadian allies to assemble and follow them at once to Tegea. When the army had proceeded as far as Orestheum they dismissed the sixth part, including the elder and the younger men, who were to keep guard at home, and arrived at Tegea with the rest of their troops. Not long afterwards the Arcadian allies appeared. They had also sent to the Corinthians, and to the Boeotians, Phocians, and Locrians, whom they summoned to meet them with all speed at Mantinea. [4] But the notice given to the allies was short, and their passage was barred by the enemies' country, which they could not easily traverse unless they waited for one another and came all together. However, they did their best. [5] The Lacedaemonians, accompanied by their Arcadian allies, invaded the territory of Mantinea, and pitching their camp near the temple of Heracles, wasted the country.