Alexander I of Macedon (fl.507-463 BC) was the king of Macedon during Xerxes' invasion of Greece, and although he was forced to serve in the Persian army, he was also willing to provide information to the Greeks (Greco-Persian Wars).
In 516 the Persian emperor Darius I the Great invaded Thrace, where he established a long-term Persian presence, although a campaign against the Scythians (c.513 BC) was less successful. In the aftermath of these campaigns he left Megabazus in command of the 80,000 Persian troops who remained in Europe. In around 507 BC Megabazus sent ambassadors to Macedon, to demand earth and water - the tradition sign of submission to the Persians. King Amyntas I, Alexander's father, agreed to submit, and held a banquets for the Persian ambassadors. This went disastrously wrong. The Persians are said to have insisted that the ladies of the court should attend the feast, and then treated them badly. Alexander ordered the ladies to withdraw, claiming that they would return after beautifying themselves. Instead he sent in a group of Macedonian youths in women's cloths, who killed the envoys.
This sort of affront would normally have led to war - the Spartan murder of Persian envoys helped trigger the Persian invasions of Greece - but in this case Alexander got away with it. Megabazus sent an army, under a general named Bubares, but Alexander gave him his sister in marriage, and was forgiven.
Soon after these events Alexander succeeded to the throne, possibly in 498 BC.
In 492 Darius sent his nephew Mardonius to invade Greece (Greco-Persian Wars). This expedition failed after his fleet was destroyed while passing around Mt. Athos, and Mardonius lost his command. During the invasion he did force Alexander to submit to him, and Macedon remained a Persian ally during Darius's and Xerxes's invasions.
In 480 BC Alexander accompanied Xerxes's army and is said to have gained the trust of Mardonius. In the aftermath of the Greek victory at Salamis, Xerxes withdrew from Greece, leaving Mardonius to command the sizable army left behind. Alexander was sent to the Athenians (then in exile on Salamis) with peace terms, which he suggested that the Athenians should accept on the grounds that they couldn't beat the Persians. The Athenians were offered autonomy, the restoration of all of their territories and the right to expand into new areas, in return for submitting to Persia and joining their military alliance. Unsurprising the Athenians turned down the offer, but they were able to use it to force the Spartans to come and fight outside the Peloponnese.
In 479 Alexander was still with the Persian army, but he was now more willing to help the Greeks. On the night before the battle of Plataea, Alexander came to the Greek camp to tell them that Mardonius was planning to fight on the following day, even though he had been unable to get good omens from the sacrifices required before battle. This might have been an attempt to gain credit with the Greeks, or possibly he was sent by Mardonius to make sure that the Greeks would remain in place and fight on the following day.
Alexander was recorded as still being alive and on the throne in 463 BC. He was succeeded as king by his son Perdiccas II.
Alexander was the member of the Macedonia royal family to compete at the Olympic Games. In order to do this he had to prove his Greek descent, Macedon then being on the very northern edge of the Greek world. He was able to claim Greek ancestry as his family claimed to be from Argos, and he tied for first place in one of the longer distance races.
During his reign Macedon increased in size. He was said to have been the monarch who first established his authority over Upper Macedonia, the northern part of the area, although effective Royal control had to wait until Philip II.