When the Persian king, Xerxes, invaded Greece in the spring of 480 BC, he did so at the head of a vast army. Once the Spartan force at Thermopylae had been defeated, his route by land to Athens was virtually undefended. Attica was seized by panic.
The Athenians sought the wisdom of the Oracle of Delphi. According to Herodotus the Oracle's response could hardly have been more negative:
'Why sit you doomed one? Fly to the ends of the earth. All is ruin for fire and headlong god of war shall bring you low.'
When the message reached Athens the popular assembly fell into uproar and another envoy was quickly dispatched to the Oracle. The second prophecy was less apocalyptic:
'Though all else shall be taken, Zeus, the all seeing, grants that the wooden wall only shall not fail.'
Argument raged as to what this 'wooden wall' could mean, with many believing it to be the thorn bushes surrounding the Acropolis. Themistocles had an answer of his own: the wooden wall, he argued, was nothing less than the fleet they had spent these last few years hurriedly constructing. He won the day and then gave the order for Athens itself to be abandoned…
The Oracle at Delphi
Themistocles' Order for Athens to be Abandoned (The Declaration of Troizen)