From Citizendium, the Citizens' Compendium
The Trojan Horse was a historically famous military deception in which a besieging army of the Greek expeditionary force hid warriors inside a tall wooden structure built to resemble a horse, and sailed away, pretending the war was over. The besieged Trojans, further deceived by a Greek left behind to plant a false story about the horse, were fooled into believing that the decade-long Trojan War was over, and wheeled the large structure inside their walls partly under the belief that doing so would be beneficial to their cause with the gods. Trojans failed to heed the warning of the savvy priest Laocoön who suspected trickery, as well as Cassandra who predicted, accurately, that the horse would be the downfall of Troy. At night, with the people of Troy asleep, select Greek fighters inside the hollowed-out belly of the horse slipped out and opened the city's gates, which allowed returning Greek shipboard warriors to swarm through the city's walls, and Troy was sacked and burned. Whether this incident happened as described in numerous Greek epic poems and Greek tragedies, and was fully described in the Roman poet Virgil's The Aeneid has never been firmly established, although there is strong evidence that there was a city of Troy in Asia Minor in northwestern Turkey. The term Trojan horse has been used in computer terminology to describe malware which tricks a user to let into one's computer, by appearing innocent, but which causes damage afterwards.
Homer's Iliad does not describe the episode of the Trojan horse, although there are references thoughout indicating that the bard Homer expected listeners to the tales to be thoroughly familiar with the basic events of the Trojan War. The strategist credited with thinking up the famous ruse, according to mythology, was the wily Odysseus, king of Ithaca. The strategy solved a puzzle preventing the Greeks, with a numerically superior force, couldn't solve on the open battlefield; since Troy had strong and tall walls, the ten-year war was essentially a stalemate.
It was necessary to persuade the Trojans to move the large wooden horse physically inside their walls. To trick them, the Greeks left behind a man prepared with an ingenious lie. The Trojans emerged from their walls to find the Greek fleet departed, with a giant wooden horse near the shore, and a man supposedly abandoned by the Greeks. His name was Sinon, according to Virgil in The Aeneid, described by the poet as a "cool intriguer". Sinon was seized by Trojan warriors and brought before King Priam. Sinon pretended to have had a falling out with the Greeks, particularly with Ulysses, who had supposedly planned to have sacrificed him; but according to the story, Sinon had escaped from the sacrifice. Sinon begins by pleading for his life:
What land, what waters, can take me now? There is nothing, nothing left for me any more, no place with the Greeks, and here are the Trojans howling for my blood.
The Trojans pitied him, and listened to his story. He establishes that he was, in fact, a Greek, but that he had had a falling out with the Greeks. Sinon explains that the Greeks were tiring of the war, but were told to sacrifice one of their own; according to Sinon's lie, the Greeks chose him for the sacrifice, but he escaped in time; hence, he didn't leave with the Greek fleet. He wins the confidence of the Trojans, who untie him and promise to spare his life. At this point, the curious Trojans ask him about the horse:
Why they have built this monstrous horse? Who made it? Who thought of it? What is it, war-machine, religious offering?
And Sinon explained that the horse was an offering to the goddess Athena or Minerva (Roman name) to appease her anger, and it was deliberately built so large that it couldn't enter the walls of Troy; but if it was damaged, harm would come to those who damaged it. He explained:
It was Calchas, again, who bade them build a mass so mighty it almost reached the stars, too big to enter through any gate, or be brought inside the walls. For if your hands should damage it, destruction, (May God avert it) would come upon the city, but if your hands helped bring it home, then Asia would be invading Greece, and doom await our children's children (ie the Greeks).
The ruse suggested to the Trojans that IF they brought the horse within their city, that this would meet the blessing of Athena, and secure for them luck for generations thereafter. Sinon did not say this explicitly; rather, his story led the Trojans to this disastrous conclusion. Accordingly, the Trojans brought the horse inside the city and had to knock down a portion of their walls for the contraption to enter.
According to the account by Virgil, the Trojan priest Laocoon and his two sons were killed by giant serpents from the sea, who thereafter slithered to the temple of Minerva, and the Trojans interpreted this, incorrectly, that they must appease Minerva by bringing the horse into the city. At night, Sinon opened the underside to the horse, allowing warriors inside including Ulysses to emerge, and they killed the Trojan guards, opened the gates, and Troy was sacked. In the Aeneid, Virgil's line Timeo Danaos et dona ferentes meaning I fear Greeks even those bearing gifts which has been paraphrased to Beware Greeks bearing gifts. King Priam's daughter Cassandra, the soothsayer of Troy, insisted that the horse would be the downfall of the city and its royal family but she too was ignored, hence their doom and loss of the war.
The Greek soldiers hidden inside the horse are thought to have included: Odysseus, Agamemnon, Acamas, Agapenor, Ajax the Lesser, Amphimachus, Antiklos, Antiphates, Cyanippus, Demophon, Diomedes, Echion, Epeius, Eumelus, Euryalus, Eurydamas, Eurymachus, Eurypylus, Ialmenus, Idomeneus, Iphidamas, Leonteus, Machaon, Meges, Menelaus, Menestheus, Meriones, Neoptolemus, Peneleus, Philoctetes, Podalirius, Polypoetes, Sthenelus, Teucer, Thalpius, Thersander, Thoas, Thrasymedes. But the list of warriors varies according to differing accounts.