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Defeat of Demetrius and Lysimachus

Seleucus nominated his son Antiochus I as his co-ruler and viceroy of the eastern provinces in 292 BC, the vast extent of the empire seeming to require a double government. In 294 BC Stratonice married her stepson Antiochus. Seleucus reportedly instigated the marriage after discovering that his son was in danger of dying of love sickness. Seleucus was thus able to get Stratonice out of the way, as her father Demetrius had now become king of Macedonia.

The alliance between Seleucus and Demetrius ended in 294 BC when Seleucus conquered Cilicia. Demetrius invaded and easily conquered Cilicia in 286 BC, which meant that Demetrius was now threatening the most important regions of Seleucus' empire in Syria. Demetrius' troops, however, were tired and had not received their payment. Seleucus, on the other hand, was known as a cunning and rich leader who had earned the adoration of his soldiers. Seleucus blocked the roads leading south from Cilicia and urged Demetrius' troops to join his side. Simultaneously he tried to evade battle with Demetrius. Finally, Seleucus addressed Demetrius personally. He showed himself in front of the soldiers and removed his helmet, revealing his identity. Demetrius' troops now started to abandon their leader en masse. Demetrius was finally imprisoned in Apameia and died a few years later in captivity.

Lysimachus and Ptolemy had supported Seleucus against Demetrius, but after the latter's defeat the alliance started to break apart. Lysimachus ruled Macedonia, Thracia and Asia Minor. He also had problems with his family. Lysimachus executed his son Agathocles, whose wife Lysandra escaped to Babylon to Seleucus.

The unpopularity of Lysimachus after the murder of Agathocles gave Seleucus an opportunity to remove his last rival. His intervention in the west was solicited by Ptolemy Keraunos, who, on the accession to the Egyptian throne of his brother Ptolemy II (285 BC), had at first taken refuge with Lysimachus and then with Seleucus. Seleucus then invaded Asia Minor and defeated his rival in the Battle of Corupedium in Lydia, 281 BC. Lysimachus fell in battle. In addition, Ptolemy had died a few years earlier. Seleucus was thus now the only living contemporary of Alexander.