An awkward hero
Miltiades is remembered as the architect of the Athenian victory at Marathon. The rest of his career fits poorly with that role, and Herodotus, who tells the story, does not smooth it.
Before Marathon he had ruled the Thracian Chersonese as a tyrant, a position his family held under Persian overlordship. He served in Darius's Scythian campaign as a subject commander. Herodotus reports that he proposed destroying the bridge over the Danube to strip the Persian army, a claim that reads as a later defence of a man who had spent years in Persian service and would have needed one.
He fled to Athens when the Ionian revolt collapsed, was prosecuted for having been a tyrant, and was acquitted.
Marathon
In 490 BCE he was one of the ten elected generals. Herodotus's account has the board divided on whether to fight, and the polemarch Callimachus casting the deciding vote after Miltiades argued for battle.
His specific tactical contribution is less clearly attested than the tradition suggests. Herodotus describes the thinned centre and reinforced wings without saying who devised them, and the sequence by which the Athenian wings turned inward to envelop the Persian centre is described as what happened rather than as what was planned. See Marathon.
After the battle
The year after Marathon he persuaded the Athenians to give him a fleet for an expedition whose purpose he would not disclose. He used it to attack Paros, apparently to settle a private grievance, failed, and was seriously injured.
He was prosecuted on his return, convicted, and fined an enormous sum. He died of his wound before paying it, and the debt fell on his son Cimon.
What this figure is useful for
Miltiades is a case study in how Athenian democracy handled successful generals: elected annually, prosecuted for failure, and given no lasting authority by victory. Marathon made him the most celebrated man in Athens and did not protect him for a year.
He is also a caution about hero-formation. The version in which a far-sighted patriot saves Greece is a construction; the man in Herodotus is an ex-tyrant with Persian service behind him who won a battle and then misused a fleet.