Xenophon Hellenica 6.5.16

[16] On the next day Agesilaus encamped at a distance of about twenty stadia from Mantinea. But the Arcadians from Tegea, a very large force of hoplites, made their appearance; they were skirting the mountains between Mantinea and Tegea, desiring to effect a junction with the Mantineans, for the Argives, who came with them, were not in full force. And there were some who tried to persuade Agesilaus to attack these troops separately; he, however, fearing that while he was marching against them the Mantineans might issue forth from their city and attack him in flank and rear, judged it best to allow the two hostile forces to come together and, in case they wished to fight, to conduct the battle in regular fashion and in the open.
  The Arcadians from Tegea had by now effected a junction with the Mantineans.