 The allied Greek land forces, which Herodotus states numbered no more than 4,200 men, had chosen Thermopylae to block the advance of the much larger Persian army.Although this gap between the Trachinian Cliffs and the Malian Gulf was only "wide enough for a single carriage", it could be bypassed by a trail that led over the mountains south of Thermopylae and joined the main road behind the Greek position.Herodotus notes that this trail was well known to the locals, who had used it in the past for raiding the neighboring Phocians.