Mardonius (d.479 BC) was a successful Persian general who helped reconcile the Ionian Greeks after the end of the Ionian Revolt, took part in Xerxes's invasion of Greece, and who was killed at the decisive battle of Plataea in 479 BC. He was a nephew of Darius I, and married to his daughter Artazostra, making him the son-in-law of Darius and the brother-in-law of Xerxes I.
In 522 a revolt broke out against Cambyses II, the Persian Emperor, who had been absent in Egypt for too long. Gaumata (or Smerdis) the Magian claimed to be Bardiya, son of Cyrus the Great and a brother of Cambyses. Cambyses died while returning from Egypt, and Darius, a prince from a different branch of the Royal Family overthrew Gaumata. He was supported by six Persians, who first realised that Gaumata wasn't the real prince. Amongst them was Gobryas, Mardonius's father. This helped establish a close connection between the two families.
In 499 the Greek cities of Asia Minor rebelled against Persian control (Ionian Revolt), receiving some aid from Athens and Eretria. After some success the revolt was put down. In 492 the young Mardonius replaced Artaphernes the Elder as satrap of Ionia, and was also given a special commission to punish Athens and Eretria for their part in the Ionian Revolt (Greco-Persian War). He was a great success in Ionia, where he replaced the previous Persian-backed tyrannies with new democratic regimes.
The expedition against Athens and Eretria was less successful. He invaded Thrace and Macedonia, and despite an initial defeat managed to conquer the Brygi. His fleet was then wrecked while passing around Mt. Athos, and as a result he was removed from his post. Darius sent another expedition against Athens in 490, but this force was defeated at the battle of Marathon. Darius died before he could launch a third invasion of Greece.
Darius's son and successor Xerxes I needed a few years to secure his empire. Once his rule was firmly established he came under pressure to resume his father's attack on Greece. Mardonius was said to have been one of the pro-war party. After making massive preparations, Xerxes invaded Greece in 480. Mardonius was one of the commanders of his army. Although Xerxes won a victory at Thermopylae and sacked Athens, the decisive battle of 480 came at Salamis, where the Greek fleet inflicted a heavy defeat on the Persians. Xerxes decided to withdraw from Greece and returned home. Mardonius was left in command of a sizable Persian army that was left in Thessaly.
In 479 Mardonius attempted to win over Athens, sending Alexander I of Macedon with a peace offer. When this failed he invaded Attica and occupied Athens in June 479. Mardonius then managed to pull the Greeks north to Plataea, in the foothills of Mt. Cithaeron. After some manoeuvring the two armies came face to face, but Mardonius then paused for ten days before finally he attacked. At first the Persians held their own, but Mardonius was fighting in the front lines of the Persian army, and he was eventually killed. With their commander gone, the Persian army collapsed and the survivors fled. This defeat forced the Persians to withdraw from mainland Greek. At about the same time the Persian forces in Asia Minor suffered a heavy defeat at Mycale, ending the Persian threat to Greece, at least for a few years.