Everyone knows the story of the Trojan horse. The Greeks hid a small force of their warriors in the belly of a great wooden horse. Then they wheeled the horse to the gates of Troy and said, "Yo, dudes, we give up. Before we take off, please accept this noble steed as our gift to you." Then the Greeks boarded their ships and sailed off.
The Trojans should have been suspicious (heck, even the Trojan priest Laocoön warned about Greeks bearing gifts), but instead they opened the gates, pushed the horse inside and proceeded to party like it was 1999. Meanwhile, the Greek fleet was hiding out at a neighboring island, waiting patiently. In the middle of the night, they sailed back to Troy and marched quietly to the gates, which had been thrown open by the soldiers hidden in the horse. The rest is blood and guts and a lot of drunken mayhem.
Today, some scientists question the validity of the story. They argue, for example, that the wooden horse may have been a battering ram used to break down the gates of Troy. But oral historians, who apparently wanted a grander tale to tell, altered the facts and injected the idea of an equine disguise. Either way, a Trojan horse has come to symbolize any form of subterfuge that causes one group of people to unwittingly invite a subversive enemy (or enemies) into a protected area.