Greek spies at Sardes
by Jona Lendering

Herodotus, The Histories 7.146-147: The story of the Greek spies

They arrived in Sardis and found out all they could about the king's army, but were caught in the process, questioned by the Persian army commanders and condemned to death. But when Xerxes was told that they were about to be executed, he disapproved of his generals' decision and sent men from his bodyguard with orders to get hold of the three spies, if they were still alive, and bring them before him. As the sentence had not yet been carried out, this was done; the spies were brought to the king, who, having satisfied himself about the reason for their presence in Sardis, instructed his guards to take them round and let them see the whole army, infantry and cavalry, and then, when they were satisfied that they had seen everything, to let them go without molestation to whatever country they pleased. 

After giving this order, he explained the purpose of it by pointing out that, if the spies had been executed, the Greeks would not have been able to learn in good time how incalculably great the Persian strength was - and the killing of three men would not have done the enemy much harm; but if, on the other hand, the spies returned home, he was confident that their report on the magnitude of the Persian power would induce the Greeks to surrender their liberty before the actual invasion took place, so that there would be no need to go to the trouble of fighting a war at all.