Darius I was the third king of the Persian Achaemenid Empire. Also called Darius the Great, he ruled the empire at its very peak, when it included much of West Asia, the Caucasus, parts of the Balkans, most of the Black Sea coastal regions, parts of the North Caucasus, Central Asia, as far as the Indus Valley in the far east, and portions of north and northeast Africa including Egypt, eastern Libya and coastal Sudan.

Darius ascended the throne by overthrowing Gaumata, the alleged usurper of Bardiya with the assistance of six other Persian noble families; Darius was crowned the following morning. The new king met with rebellions throughout his kingdom and quelled them each time. A major event in Darius’s life was his expedition to punish Athens and Eretria for their aid in the Ionian Revolt against subjugation in Greece. Although ultimately ending in failure at the Battle of Marathon, Darius succeeded in the re-subjugation of Thrace, expansion of the empire through the conquest of Macedon, the Cyclades, and the island of Naxos, and the sacking and enslavement of the city of Eretria.

Darius organized the empire by dividing it into provinces and placing satraps to govern it. He organized a new uniform monetary system, along with making Aramaic the official language of the empire. He also put the empire in better standing by building roads and introducing standard weights and measure. Through these changes, the empire was centralized and unified. Darius also worked on construction projects throughout the empire, focusing on Susa, Pasargadae, Persepolis, Babylon and Egypt. He had the cliff-face Behistun Inscription carved to record his conquests, an important testimony of the Old Persian language.

Darius’s European expedition was one of the major events of his reign, which began with the invasion of Thrace. Darius also conquered many cities of the northern Aegean, Paeonia, while Macedonia submitted voluntarily becoming a vassal kingdom. He then left Megabyzus to conquer Thrace, returning to Sardis to spend the winter. The Greeks living in Asia Minor and some of the Greek islands had submitted to Persian rule already by 510 BCE. Nonetheless, there were certain Greeks who were pro-Persian, although these were largely based in Athens. To improve Greek-Persian relations, Darius opened his court and treasuries to those Greeks who wanted to serve him. These Greeks served as soldiers, artisans, statesmen and mariners for Darius. However, the increasing concerns amongst the Greeks over the strength of Darius’s kingdom along with the constant interference by the Greeks in Ionia and Lydia were stepping stones towards the conflict that was yet to come between Persia and certain leading Greek city states.

When Aristagoras organized the Ionian Revolt, Eretria and Athens supported him by sending ships and troops to Ionia and by burning Sardis. Persian military and naval operations to quell the revolt ended in the Persian reoccupation of Ionian and Greek islands, as well as the re-subjugation of Thrace and the conquering of Macedonia in 492 BC under Mardonius. Macedon had been a vassal kingdom of the Persians since the late 6th century BC, but remained having autonomy. Mardonius’ 492 campaign made it a fully subordinate part of the Persian kingdom. These military actions as a direct response to the revolt in Ionia commenced the beginning of the First Persian invasion of (mainland) Greece. At the same time, anti-Persian parties gained more power in Athens, and pro-Persian aristocrats were exiled from Athens and Sparta. Darius responded by sending troops led by his son-in-law across the Hellespont. However, a violent storm and harassment by the Thracians forced the troops to return to Persia. Seeking revenge on Athens and Eretria, Darius assembled another army of 20,000 men under his Admiral, Datis, and his nephew, Artaphernes, who met success when they captured Eretria and advanced to Marathon. In 490 BCE, at the Battle of Marathon, the Persian army was defeated by a heavily armed Athenian army, with just over 9,000 men who were supported by 600 Plataeans, 1,000 soldiers from each of eleven Greek city-states and 10,000 lightly armed soldiers led by Miltiades.

The defeat at Marathon marked the end of the first Persian invasion of Greece. Darius began preparations for a second force which he would command, instead of his generals; however, before the preparations were complete, Darius died, leaving his empire in the hands of Xerxes.