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Seleucus needed the assistance of Ptolemy who was the de-facto monarch in Egypt. The support that the Macedonian ruler of Egypt, especially the veterans he supplied were crucial in the defeat of Antigonus and his son Demetrius. Ptolemy wanted Seleucus to distract Antigonus army in the east so that the Antagonids would not invade his Egyptian territories. The Babylonian War also allowed Ptolemy to survive an onslaught on his kingdom. The support that Ptolemy afforded Seleucus cemented the alliance between the two Macedonians. Earlier Seleucus had served under Ptolemy as his admiral and the men had a close relationship. This relationship was to prove crucial in the years ahead. Their understanding was based on the acceptance that both had legitimate spheres of influence, Seleucus in the East and Ptolemy in Egypt, this was crucial as it effectively meant that there would be no further attempts to reunite the Empire of Alexander the Great. The two rulers’ understandings were also to be very important in the fourth Diadochi war.

==Victory in the Fourth Diadochi War==

Antigonus was not deterred by his defeat outside the walls of Babylon. In 301 BCE he ordered his son Demetrius to seize those parts of Greece, which were part of the Kingdom of Cassander. The other Diadochi kings including Seleucus formed a coalition to defend their interests and to ensure that the Antigonid’s did not become too powerful. The armies of Ptolemy and Seleucus marched into modern Turkey and they confronted a huge army under the leadership of Antigonous<ref> Davis, Paul K.. 100 Decisive Battles from Ancient Times to the Present: The World’s Major Battles and How They Shaped History (Oxford, Oxford University Press, 1990), p. 134</ref>. The two armies clashed near the village of Ipsus, today in southern Turkey. The battle was a decisive victory for Seleucus and his allies. Antigonus was killed during this battle and his son escaped to Greece with a small force. The alliance partitioned the territory of Antigonous and Seleucus secured Syria and Lebanon and his Empire stretched from the Mediterranean to the Hindu Kush. The army of Seleucus was critical to the defeat of Antigonus, the war elephants he had received from Indian proved decisive<ref>Davis, p. 167</ref>. However, the foundation for the victory of Ipsus had been established during the Babylonian War. Because Antigonus was unable to dislodge Seleucus from Babylon, the latter could create a huge empire in the east. This made him the most powerful of all the Hellenistic kings and that he could raise a huge army that made the defeat of Antigonus almost inevitable. The victory of the founder of the Seleucid dynasty in Babylon meant that Antigonus was doomed and that there would be no further attempt to resurrect the Empire of Alexander. Soon after Ipsus, there emerged three successor kingdoms, the Seleucid Empire, the Ptolemaic Empire, based in Egypt and the Kingdom of Macedonia. This led to a long period of stability and peace which was a crucial factor in the brilliant flowering of Hellenistic culture.