History of the Peloponnesian War

At daybreak the Boeotians joined him. Having determined to relieve Megara, whose danger they considered their own, even before hearing from Brasidas, they were already in full force at Plataea, when his messenger arrived to add spurs to their resolution; and they at once sent on to him two thousand two hundred heavy infantry, and six hundred horse, returning home with the main body.

The whole army thus assembled numbered six thousand heavy infantry. The Athenian heavy infantry were drawn up by Nisaea and the sea; but the light troops being scattered over the plain were attacked by the Boeotian horse and driven to the sea, being taken entirely by surprise, as on previous occasions no relief had ever come to the Megarians from any quarter.