Demetrius I (Demetrius Poliorcetes)dĭmēˈtrēəs pŏlˌēôrsēˈtēz, c.337–283 B.C., king of Macedon. The son of Antigonus I, he proved himself a very able commander in his father's wars, particularly against Ptolemy I. Though Ptolemy defeated him at Gaza in 312 B.C., Demetrius was able to expel Cassander from Athens; he then defeated Ptolemy off Salamis and took Cyprus. Although he had huge armaments, including new weapons of assault, he failed (305 B.C.) to take Rhodes by sea. When Cassander, Seleucus I, and Lysimachus, fearing the power of Antigonus, allied themselves against him, Antigonus and Demetrius were badly defeated in the battle of Ipsus in 301 B.C., and Antigonus was killed. Demetrius later became reconciled with Seleucus I and regained Athens for himself in 295 B.C. In order to obtain the throne of Macedon he murdered his competitors, including the sons of Cassander, and succeeded (294 B.C.) to the throne. He had his father's ambition to conquer all Asia, but his enemies united against him, and when Lysimachus and Pyrrhus invaded Macedonia he was forced (285 B.C.) to take refuge with Seleucus, who held him until he died. His son, Antigonus II, made good his claim to the throne of Macedon.
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