The Babylonian War was a conflict fought between 311–309 BC between the Diadochi Antigonus Monophtalmus and Seleucus, ending in a victory for the latter. The conflict ended any possibility of restoration of the empire of Alexander the Great, a result confirmed in the Battle of Ipsus. It also marked the infancy of the Seleucid Empire by giving Seleucus control over the eastern satrapies of Alexander's former empire.
Belligerents Antigonid dynasty Seleucid Empire Commanders and leaders Antigonus I Monophthalmus Demetrius Poliorcetes Seleucus I Nicator
Preliminaries After the death of Alexander the Great on 11 June 323 BC, his empire disintegrated. Officers who were trying to save it were defeated during the First War of the Diadochi. During the Second War of the Diadochi, the power of Antigonus I Monophthalmus, who had created a state of his own in Anatolia and Syria, was growing; this caused alarm among the other generals, but in the Third War of the Diadochi, Antigonus managed to keep Ptolemy I Soter of Egypt and Cassander of Macedon in check. In December 311, the warring parties concluded the Peace of the Dynasts, and recognized each other. The only ruler who was excluded was Seleucus I Nicator. Antigonus had expelled Seleucus, satrap of Babylonia, in 316,[1] but Ptolemy had given him an army, which he now used to return to his satrapy.[2] Campaigns Seleucus, reinforced with Macedonian veterans from Harran, reached his former capital Babylon in the second half of May 311.[3] He was soon recognized as the new ruler.[4] Only the fortress remained occupied by a garrison loyal to Antigonus. Seleucus now built a dam in the Euphrates and created an artificial lake; in August, he suddenly broke the dam, and a flood wave destroyed the walls of the fortress.[5] Antigonus' satraps in Media and Aria, Nicanor and Euagoras, now decided to intervene with an army of 10,000 infantry and 7,000 horsemen, but Seleucus and an army of 3,000 infantry and 400 cavalry had been waiting for them near the Tigris since September 311.[6] By hiding his men in one of the marshes and attacking by night, Seleucus was able to defeat the Macedonian soldiers in the army of Nicanor and Euagoras, after which the Iranian soldiers decided to side with the ruler of Babylonia (November 311).[7] Without any problems, Seleucus could move through the Zagros Mountains, occupy Ecbatana (the capital of Media), and continue to Susa (the capital of Elam). He now controlled southern Iraq and the greater part of Iran. News of the defeat of Nicanor and Euagoras must have reached Antigonus at about the time of his signing the Peace of the Dynasts (December 311). He ordered his son Demetrius Poliorcetes to restore order; he arrived in the early spring of 310, when Seleucus was still in the east. Although Demetrius managed to enter Babylon, he was not able to cope with the resistance that Seleucus' adherents were able to organize, and he returned to Syria without having achieved his goal.[8] His father Antigonus tried again in the autumn of 310, and also managed to enter Babylon,[9] but was forced to leave the city in March 309.[10] Returning to the northwest, he met the army of Seleucus, who ordered his soldiers to have their meal during the night, attacked Antigonus' soldiers while they were having breakfast, and won a decisive victory.[11]

Significance:
Antigonus retreated and accepted that Babylonia, Media, and Elam belonged to Seleucus. The victor now moved to the east and reached the Indus valley, where he concluded a treaty with Chandragupta Maurya. The Mauryan emperor received the eastern parts of the Seleucid Empire, which included Afghanistan, Pakistan and west India, and gave Seleucus a formidable force of five hundred war elephants.[12] By adding all of Iran and Afghanistan, Seleucus became the most powerful ruler since Alexander the Great.[13] Restoration of Alexander's Empire was, after the Babylonian War, no longer possible. This outcome was confirmed in the Fourth War of the Diadochi and the Battle of Ipsus (301).