Herodotus opens with an explanation that his goal, in setting down his Histories, is twofold. First, he preserves the deeds of men that might otherwise be forgotten, and should not be forgotten because they were cool and impressive. But second and more importantly, he wants to ensure that the cause of the war, the massive destructive horrible thing that he lives through, will never be lost. Learn from the Second Persian War, Herodotus is saying. Otherwise this will only happen again.
Fortunately, as a result of Herodotus’s effort, there have been zero wars since 470 BCE. Thanks, Herodotus!
Originally conflict began with the Phoenicians, who in addition to inventing fighting (bunch of jerks) traveled from their original homeland on the western coast of Africa. They went up the Gulf of Suez to found a trading network that ran all through the Mediterranean, centered on their cities in Lebanon. Once upon a time, more or less, a Phoenician trading ship landed at the city of Argos, at that time in all points the first of the States within that land which is now called Hellas. Hellas, just as a reminder, was Herodotus’s au courant term for what we now call Greece. Argos still exists, with about 30 000 people living there, but it isn’t the cultural capital of Greece the way it was back in the day. The king of Argos, Inachos, had a daughter named Io whom the Phoenican traders abducted. They set up their wares on the deck of their ship, and invited the women of Argos to come up and shop, and then boom, they took off with the women aboard. Most of the women escaped, probably by jumping overboard, but Io and a nontrivial number of others were grabbed by the sailors and restrained and carted off to Egypt.
At this point Herodotus drops this particular narrative thread, so let’s all hope that Io escaped her captors and married a handsome Egyptian prince (or lovely Egyptian princess, or one of each, hey, whatever floats Io’s boat). But she never returned to Argos; the damage was done, and a group of Greeks were spurred to revenge. Herodotus cannot confirm that these Greeks were Inachos and his friends; they may have been an unrelated but sympathetic group from Crete. This team sailed to Tyre, in Lebanon, which at the time was a major Phoenician city. There they abducted Europa, the princess of Tyre, through a scheme Herodotus can’t identify. This should have been the end of it, but then the Greeks took it too far: they abducted and apparently raped another Phoneician princess, Medea of Colchis. Colchis was located in modern-day Georgia, which is to say, quite a ways from Lebanon, so there was really no excuse for this heinous act.
The king of Colchis sent a stern letter to Greece, which was answered with a big raspberry. And thus Phoenician-Greek relations soured, though things wouldn’t get really bad for another generation. Some number of years later, Alexander, son of Priam and the young prince of the Phoenician city of Troy, came of marrying/abducting age. He set his sights on a Greek princess, Helen of Lacedemon. That she was married to someone else: not a concern of his!
Alexander carried off Helen, as Herodotus puts it, and the Greeks refused to let this lie. They invented a wholly new thing: massive armed invasion! Up to this point warfare consisted solely of stealing women, but the Greeks took it to a new level. They assembled a fleet of ships, sailed to Troy, and tore that sucker down! No more Troy! Troy was a thing of the past; they destroyed the dominion of Priam. The Persians point to this as the point when the Greeks and the Persians became permanent, implacable enemies, because the Persians considered the Phoencians to be a subset of Persians.
The Phoenicians themselves claimed that Io, the supposedly abducted Argive princess who set all this off, actually went willingly because the captain of the Phoenician trading ship knocked her up, and the two of them went off to Egypt and lived happily ever after with a passel of half-Greek, half-Phoenician children. Ianchos was terrible father and Io was afraid to tell him she’d gotten Teen Pregnant, is why they had to arrange the fake kidnaping and all.
Herodotus has no dog in this particular fight. People have been treating one another badly for basically ever, he says, and the Greeks did things just as bad as the non-Greeks. But he has to start his story somewhere, and with all this Trojan War stuff as prelude, he wants to open with the first non-Greek who started the ball rolling, and move forward from there.