They had evidently supposed that some large force was on its way to Thermopylæ at the time when the disaster took place, and had expected to find, at any rate, an army in Bœotia. What they actually discovered was that the Peloponnesians were fortifying the Isthmus, and were concentrating the defence on that point. The Peloponnesian party had at last shown its hand. Thermopylæ had been a mere pretence, an attempt to purchase the aid of the Athenian fleet, and, possibly, of the Northern States, at the cheapest rate possible,—at a much cheaper rate, indeed, than the price which had been actually paid. The idea of the Peloponnesians at this time evidently was that the Athenians might be led to suppose that, as the defence of so strong a position as Thermopylæ had been unsuccessful, no other position north of the Isthmus could possibly be regarded as tenable. That which the Athenian commanders did recognize was that, as in their absence at Artemisium no sort of attempt had been made to provide for the defence of either the passage by Lake Kopais or of the Kithæron-Parnes line of mountains, the military situation must be, perforce, accepted, and their efforts must be concentrated on inducing the allies to take a sound view of the necessities of the naval position. In its main outlines it was plain to the dullest intelligence. The defence of the Isthmus was as much, if not more, dependent on the fleet than that of Thermopylæ. If the Persians could once land troops in Argolis, the position at the Isthmus was fatally turned. The saving factor in the situation was that the fleet was just as necessary an adjunct for the Peloponnesian as for the Northern policy. But when it became a question how the fleet should act as an aid to the defence of the Isthmus, the decision was by no means so clear. The fighting on the last day at Artemisium had shown the danger of meeting a greatly superior force on open water, where the smaller fleet could be outflanked; and the coast in the immediate neighbourhood of the Isthmus itself afforded no position which, if taken up, would guarantee the Greek fleet from attack on either flank. That was evidently the ruling consideration which decided the Greeks, on the recommendation of Themistocles, to take up their station in Salamis strait But even after that position was taken up, Themistocles must have been painfully conscious that, if the Persians chose to ignore the Greek fleet at Salamis, and to sail straight to the Isthmus, the situation would be dangerous to the last degree.