The Greco-Persian war is actually launched because of a dream. Well, sort of. See, 
the Athenians insulted Darius, Xerxes’ father. Darius swore revenge, but died before he got a chance to see to it (largely because of an Egyptian rebellion), so he made Xerxes promise to take care of it. So one of the first orders of business was deciding whether to go to war. Xerxes’ cousin and close friend, Mardonius, was the loudest voice in favor of war. His uncle, Artabanas (Darius’ younger, full brother) was the sole voice of dissent.
Now, Xerxes was not a temperate man by any account. He got royally ticked at his uncle for daring to stand up in court and say, “Look, kingy, your cousin’s a fool and you’re far too wise to listen to him. Let sleeping dogs lie. Stay here, don’t worry with Greece.” He more or less told off his uncle and retired to his rooms to rant in peace.
Of course, once his temper cooled, Xerxes was actually a man of fairness, and he realized that old Artabanas had a good point. He decided that in the morning, he’d announce that, in spite of what he said the day before, they weren’t going to war. But that night he had a dream. A handsome man appeared to him, his figure rather fierce, and taunted Xerxes for shying away from greatness. Xerxes awoke a little disturbed, but he dismissed it. I mean, the Persians were academic, scholarly folks—he knew that dreams were just the mind working through things, so why let it get to him?
He announced the change of plans. The people rejoiced. Laughter, feasting, yada yada. He went to bed again. Dreamed the same thing again and awoke trembling and fearful, convinced this man in his dream was his god, and that the god wanted him to go to war. Still . . . weird. So he went to Artabanas. Said, “Uncle, I keep having this dream, and it’s really freaking me out. So I want you to put on my robes, go sit on my throne, then go sleep in my bed, and we’ll see if the god appears to you in a dream too.”
Artabanas rolls his eyes (you could totally see him doing it in Herodotus;-) and said, “Oh, nephew. You know it doesn’t work that way. Dreams are just the work of the imagination. And even if it were really the god, he’s not going to mistake me for you. I mean, come on.”
Insert Xerxes folding his arms over his chest, rather menacing.
Artabanas probably sighed. “Okay, okay, I’ll humor you. Given that you’re the king and all.” So he did as Xerxes recommended.
And dreamed. Though, no, they didn’t fool their god. He appeared to Artabanas and demanded, “What do you think you’re doing, dissuading the king from my will? I ought to . . .” and he lunged at him with red-hot pokers aimed at his eyes.
Artabanas awoke screaming and told Xerxes, “Go to war. Yep. That’s my new advice.”
So Xerxes announced another change of plans. And the people rejoiced. (The people tended to rejoice no matter what Xerxes said. He was apparently a major hunk, one of the tallest men in court, well-built, and charming in the extreme. Ah . . . great fodder for a hero!)
(Photo is of a frieze from Susa, depicting Persian soliders on glazed bricks.)