Wooden bridges, it transpires, have a pretty short lifespan when you try to march the world’s largest army across them in a storm. A thousand Persian soldiers all try to invent armored swimming. They fail.
“Christ,” says Xerxes. “Surely my invasion of Greece can suffer no more humiliating setback!”
“A floating bridge–” begins Harpalus.
“Fine, whatever!” says Xerxes. “I’m going to whip this stupid strait with a hot iron while my generals call it names!”
Then they do that. I’m serious, look it up.
The Hellespont is largely unaffected by the whipping, but some of the name-calling cuts pretty deep.