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

71. Before they had actually closed a thought occurred to Agis. All armies, when engaging, are¹ apt to thrust outwards their right wing; and either of the opposing forces tends to outflank his enemy's left with his to outflank his enemy's left with his own right, because every soldier individually fears for his exposed side, which he tries to cover with the shield of his comrade on the right, conceiving that the closer he draws in the better he will be protected. The first man in the front rank of the right wing is originally responsible for the deflection, for he always wants to withdraw from the enemy his own exposed side, and the rest of the army, from a like fear, follow his example. In this battle the line of the Mantineans, who were on the Argive right wing, extended far beyond the Sciritae: [2] and still further, in proportion as the army to which they belonged was the larger, did the Lacedaemonians and Tegeans on the Lacedaemonian right wing extend beyond the Athenian left. [3] Agis was afraid that the Lacedaemonian left wing would be surrounded, and, thinking that the Mantineans outflanked them too far, he signalled to the Sciritae and the old soldiers of Brasidas to make a lateral movement away from his own division of the army, and so cover the line of the Mantineans: to fill up the space thus left vacant he ordered Hipponoidas and Aristocles, two of the polemarchs, to bring up their two divisions from the right wing, thinking that he would still have more troops than he wanted there, and that he would thus strengthen that part of his line which was opposed to the Mantineans.