DEMETRIUS I., king of Macedonia, a son of Antigonus and Stratonice, surnamed Poliorcetes, or the Besieger. Both father and son play an important part in the vicissitudes of the Macedonian empire after the death of Alexander the Great. Demetrius grew up to be a beautiful young man, reared in the fulness of the new Macedonian life, devoted to Greek science, and inspired with an eager ambition to rival the ancient heroes of his race. He united with these lofty aims a love of Oriental magnificence which formed at once the chief splendour and the principal ​weakness of his Macedonian prototype. At the age of twenty-two he was sent by his father against Ptolemy, who had invaded Syria; he was totally defeated near Gaza, but soon repaired his loss by a victory which he obtained over Cilles, in the neighbourhood of Myus. After conducting an expedition against Babylon, and engaging in several campaigns against Ptolemy on the coasts of Cilicia and Cyprus, Demetrius sailed with a fleet of 250 ships to Athens, and restored the Athenians to liberty, by freeing them from the power of Cassander and Ptolemy, and expelling the garrison which had been stationed there under Demetrius Phalereus. After this successful expedition he besieged and took Munychia, and defeated Cassander at Thermopylae. His reception at Athens, after these victories, was attended with the greatest servility; and under the title of "The Preserver" the Athenians worshipped him as a tutelary deity. In the next campaign he defeated Menelaus by land, and completely destroyed the naval power of Ptolemy. After an interval spent at Cyprus, he endeavoured to punish the Rhodians for having deserted his cause; and his ingenuity in devising new instruments of siege, in his unsuccessful attempt to reduce the capital, gained him the appellation of Poliorcetes. He returned a second time to Greece as liberator. But traces of Oriental despotism showed themselves, and the licentiousness and extravagance of Demetrius made the Athenians regret the government of Cassander. He soon, however, roused the jealousy of the successors of Alexander; and Seleucus, Cassander, and Lysimachus united to destroy Antigonus and his son. The hostile armies met at Ipsus, 301 B.C.

Antigonus was killed in the battle, and Demetrius, after sustaining a severe loss, retired to Ephesus. This reverse of fortune raised him many enemies; and the Athenians, who had lately adored him as a god, refused even to admit him into their city. But he soon afterwards ravaged the territory of Lysimachus, and effected a reconciliation with Seleucus, to whom he gave his daughter Stratonice in marriage. Athens was at this time oppressed by the tyranny of Cassander; but Demetrius, after a protracted blockade, gained possession of the city, and pardoned the inhabitants their former misconduct. The loss of his possessions in Asia recalled him from Greece; and he established himself on the throne of Macedonia by the murder of Alexander, the son of Cassander, 294 B.C. But here he was continually threatened by Pyrrhus, who took advantage of his occasional absence to ravage the defence less part of his kingdom; and at length the combined forces of Pyrrhus, Ptolemy, and Lysimachus, assisted by the disaffected among his own subjects, obliged him to leave Macedonia after he had sat on the throne for seven years. He passed into Asia, and attacked some of the provinces of Lysimachus with varying success; but famine and pestilence destroyed the greater part of his army, and he retired to the court of Seleucus to seek support and assistance. Here he met with a kind reception; but, nevertheless, hostilities soon broke out; and after he had gained some advantages over his son-in-law, Demetrius was totally forsaken by his troops in the field of battle, and became an easy prey to the enemy. His son Antigonus offered Seleucus all his possessions, and even his person, in order to procure his father's liberty; but all proved unavailing, and Demetrius died in the fifty-fourth year of his age, after a confinement of three years, 284 B.C. His remains were given to Antigonus, honoured with a splendid funeral at Corinth, and thence conveyed to Demetrias. His posterity remained in possession of the Macedonian throne till the time of Perseus, who was conquered by the Romans. See Macedonia.