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. Here the Boeotians were in their turn charged and engaged by the Athenian horse, and a cavalry action ensued which lasted a long time, and in which both parties claimed the victory. The Athenians killed and stripped the leader of the Boeotian horse and some few of his comrades who had charged right up to Nisaea, and remaining masters of the bodies gave them back under truce, and set up a trophy; but regarding the action as a whole the forces separated without either side having gained a decisive advantage, the Boeotians returning to their army and the Athenians to Nisaea.