There is one crucial detail about the Trojan Horse which often gets pushed aside, but it makes the story much more convincing:

Sinon, a Greek warrior left behind by the rest of the Greek army.

When the Trojans realized the Greek ships were gone, but something else was left behind, they found this fellow tied to the legs of the wooden horse. Sinon was a mediocre warrior, but had a cunning tongue instead.

Interrogating him after serious threats of violence (it’s unclear whether or not they cut off his nose), Sinon told the Trojans how the Greeks had grown weary of the conflict; and about how he’d had a quarrel with Odysseus, which left him stranded there, branded as a traitor. And how that wooden horse was an offering to the gods, for a safe trip home.

“But why”, the Trojan priests asked, “craft a horse so beautiful, yet so big and unwieldy? Wouldn’t a natural-sized one work just as well?”

“Ah! The Greeks sought great favor for their trip home, so it had to be beautiful and well-made. But they feared how Troy could bring it home, stealing from them the favor of the gods, so it was crafted larger than the Trojan gates!”

Predictably, the Trojans now wanted to bring in the horse, just to spite the Greeks. The priest Laocoön spoke against bringing in the Horse, and in some versions he threw a spear at it, but the Greek gods sent serpents to kill him, and so let the fate of Troy be fulfilled.

Taking Laocoön’s punishment as a sign in favor of keeping the Horse, the Trojans deliberately removed the top of their gates (the lintel) to make way for it, hoping to bring the blessing of the gods to them, while leaving Sinon behind. Sinon sent up a fire signal to the Greeks, hiding in the close-by island of Tenedos, informing them that the Trojans took the bait and now Greeks could invade Troy.

Probably the first case of Reverse Psychology in literature.

EDIT: Some people are reminding me in the comments about how most of the stories about the Trojan War are fictional.

I mean… really? Of course we know that. The point is: even a fictional story needs attention to details, and Sinon makes a relevant contribution to explain why the Trojans opened their gates. That’s why I decided to write this answer about him.