Early career
Demetrius served with his father, Antigonus I Monophthalmus, during the Second War of the Diadochi. He participated in the Battle of Paraitakene where he commanded the cavalry on the right flank. Despite the Antigonid left flank, commanded by Peithon, being routed, and the center, commanded by Antigonus, being dealt heavy losses at the hands of the famous Silver Shields, Demetrius was victorious on the right, and his success there ultimately prevented the battle from being a complete loss.
Demetrius was again present at the conclusive Battle of Gabiene. Directly after the battle, while Antigonus held the betrayed Eumenes, Demetrius was one of the few who implored his father to spare the Greek successor's life.
At the age of twenty-one he was left by his father to defend Syria against Ptolemy the son of Lagus. He was defeated at the Battle of Gaza, but soon partially repaired his loss by a victory in the Battle of Myus, against a general of Ptolemy, Cilles. In the spring of 310, he was soundly defeated when he tried to expel Seleucus I Nicator from Babylon; his father was defeated in the autumn. As a result of this Babylonian War, Antigonus lost almost two thirds of his empire: all eastern satrapies fell to Seleucus.
After several campaigns against Ptolemy on the coasts of Cilicia and Cyprus, Demetrius sailed with a fleet of 250 ships to Athens. He freed the city from the power of Cassander and Ptolemy, expelled the garrison which had been stationed there under Demetrius of Phalerum, and besieged and took Munychia (307 BC). After these victories he was worshipped by the Athenians as a tutelary deity under the title of Soter (Greek: Σωτήρ, "Saviour"). At this time Demetrius married Eurydike, an Athenian noblewoman who was reputed to be descendant from Miltiades; she was the widow of Ophellas, Ptolemy's governor of Cyrene. Antigonus sent Demetrius instructions to sail to Cyprus and attack Ptolemy's positions there.
Demetrius sailed from Athens in the spring of 306 BC and in accordance with his father's orders he first went to Caria where he summoned the Rhodians in an unsuccessful attempt to support his naval campaign. In the campaign of 306 BC, he defeated Ptolemy and Menelaus, Ptolemy's brother, in the naval Battle of Salamis, completely destroying the naval power of Ptolemaic Egypt. Demetrius conquered Cyprus in 306 BC, capturing one of Ptolemy's sons. Following the victory, Antigonus assumed the title "king" and bestowed the same upon his son Demetrius. In 305 BC, he endeavoured to punish the Rhodians for having deserted his cause; his ingenuity in devising new siege engines in his (ultimately unsuccessful) attempt to reduce the capital gained him the title of Poliorcetes. Among his creations were a battering ram 180 feet (55 m) long, requiring 1000 men to operate it; and a wheeled siege tower named "Helepolis" (or "Taker of Cities") which stood 125 feet (38 m) tall and 60 feet (18 m) wide, weighing 360,000 pounds (160,000 kg). After Demetrius failed to conquer Rhodes, the weapons were abandoned, and their bronze was used by the Rhodians to construct the Colossus of Rhodes.
In 304 BC, he returned a second time to Greece as liberator, and reinstated the Corinthian League, but his licentiousness and extravagance made the Athenians long for the government of Cassander. Among his outrages was his courtship of a young boy named Democles the Handsome. The youth kept on refusing his attention but one day found himself cornered at the baths. Having no way out and being unable to physically resist his suitor, he took the lid off the hot water cauldron and jumped in. His death was seen as a mark of honor for himself and his country. In another instance, Demetrius waived a fine of 50 talents imposed on a citizen in exchange for the favors of Cleaenetus, that man's son. He also sought the attention of Lamia, a Greek courtesan. He demanded 250 talents from the Athenians, which he then gave to Lamia and other courtesans to buy soap and cosmetics.
He also roused the jealousy of Alexander's Diadochi; Seleucus, Cassander and Lysimachus united to destroy him and his father. The hostile armies met at the Battle of Ipsus in Phrygia (301 BC). Antigonus was killed, and Demetrius, after sustaining severe losses, retired to Ephesus. This reversal of fortune stirred up many enemies against him—the Athenians 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 Lachares—a popular leader who made himself supreme in Athens in 296 BC—but Demetrius, after a protracted blockade, gained possession of the city (294 BC) and pardoned the inhabitants for their misconduct in 301 BC in a great display of mercy, a trait Demetrius highly valued in a ruler.
After Athens' capitulation, Demetrius formed a new government which espoused a major dislocation of traditional democratic forms, which anti Macedonian democrats would have called oligarchy. The cyclical rotation of the secretaries of the Council and the election of archons by allotment, were both abolished. In 294/3 - 293/2 BC, two of the most prominent men in Athens were designated by the Macedonian king, Olympiordoros and Phillipides of Paiania. Their appointment by the King is implied by Plutarch who says that "he established the archons which were most acceptable to the Demos."