Near this point in Mid Bœotia the invader from the north is faced by a barrier of a composite character, which, though not homogeneous, is one of the most effective in the whole of Greece. From the west the spurs of Helicon came down to the marshy expanse of Lake Kopais, a hundred square miles of shallow reed-covered mere, when the winter rains had raised it to its full height. From Kopais to the Euripus extended a narrow mountainous region, never rising to any great height, but exceedingly rough and broken, and affording no way of passage through it better than a mere track. The consequence of this natural formation of the country was that an invader from the north was confined in his advance to the narrow breadth of land between Helicon and Kopais. So this mere ribbon of country became the great battle-ground of Greece, and on it the fate of the land was again and again decided at Chæronea, Koronea, and elsewhere. This part of the great northern route may be said to end at Thebes, which town, like Larisa and Lamia, was not merely a point where important roads converged, but also a point which must be passed in the passage from the North. To it came the great road which has just been described, and also a hardly less important road, which ran from Chalkis on the Eastern sea, and was continued through it to Kreusis on the Corinthian Gulf. Thebes was thus one of the great strategic positions in Greece. Xerxes’ main object after taking Thermopylæ was to strike at Attica. Two minor objects had to be attained by the way; the punishment of the Phocians, and the plunder of Delphi. The Phocians were the only people in the north, so Herodotus says, who did not medize. The Bœotians, wavering if not disloyal before, doubtless medized _en masse_ after Thermopylæ had shown them the insincerity of the Council at the Isthmus with regard to the defence of the north. It is difficult to account for the Phocian attitude. ◆H. viii. 30.◆ Herodotus says they were simply actuated by the desire to take the opposite side to the Thessalians. It is possible that they dared not trust to the mercy of the Persian, seeing they had taken part in the defence of Thermopylæ. Besides, their own deadly foes were in the enemy’s camp. Whatever their motive was, ◆H. viii. 30, _ad fin._◆ they took the moral ground of refusing to be traitors to their fatherland, and rejected the offer of the Thessalians to save them at a price. The first step in the march south which Herodotus describes is the invasion of Phocis. ◆H. viii. 31.◆ A body of troops, under the guidance of the Thessalians, made their way into Doris, that dreary little upland plain at the head of the Kephissos basin. The route taken must have been by the Asopos defile. Doris was left unharmed. It had medized; and, besides, the Thessalians intervened on its behalf. Its poverty may have had something to do with its salvation on this occasion.[148]