Diadochi ("successors"): name of the first generation of military and political leaders after the death of the Macedonian king and conqueror Alexander the Great in 323 BCE. To settle the question whether his empire should disintegrate or survive as a unity, and, if so, under whose rule, they fought several full-scale wars. The result, reached by 300, BCE, was a division into three large parts, which more or less coincided with Alexander's possessions in Europe, Asia, and Egypt.
During the next quarter of a century, it was decided whether these states could endure. As it turned out, there were no great territorial changes, although there were dynastic changes. After 280, the period of state-forming came to an end with three great states: Antigonid Macedonia, Ptolemaic Egypt, and the Seleucid kingdom in Asia.
After the Second Diadoch War, Antigonus Monophthalmus was sole ruler in the east, and the strongest of the Diadochi. Ptolemy was alarmed by the growth of his power, knowing that he would be unable to retain the independence of Egypt against the united forces of Asia. We have already seen that the conflict between Antigonus and Ptolemy was not new (above).
The ruler of Egypt warned Cassander and Lysimachus, the satrap of Thrace. In the autumn, the three men concluded an alliance against Antigonus, and sent an ultimatum to Antigonus, which reached him in the winter of 316: all money had to be redistributed and he had to give up his conquests. Of course, the man who had in three years' time conquered everything between the Aegean Sea and Iran, was unwilling to give in. A new war broke out: the Third Diadoch War (spring 315).
Antigonus immediately seized the initiative. He invaded Syria to secure Phoenicia with its naval resources, which were needed for anyone who had to invade the Aegean world or Egypt. In the summer, he laid siege to Tyre, which had become independent but was supported by Ptolemy (text). The defenders withstood their enemies for a long time, which offered Seleucus, who now served as Ptolemy's admiral, an opportunity to conquer Cyprus. He continued to the Aegean Sea, where he visited Miletus and the oracle of the Branchidae, which greeted him as 'king'.
At the same time, Antigonus demanded that Cassander explained what had happened to Alexander's mother Olympias (who had already been murdered), his wife Roxane and his son, the young Alexander (who had not been seen in public for some time). Of course, Antigonus allied himself to Polyperchon, who still controlled part of the Peloponnese and would prevent Cassander's crossing to Asia; and finally, Antigonus repeated the proclamation that Polyperchon had made four years before: that the Greeks were to be 'free and autonomous'. This was an extremely clever move, because Ptolemy, who wanted to act as protector of Greece as well, was now forced to guarantee its freedom as well, and thus act against Cassander's wishes (text). Antigonus' final move against the ruler of Macedonia was the creation of a federation of the island states in the Aegean Sea, which he could use when he attacked Cassander at home (the Nesiotic League).
In November 315, the alliance with Polyperchon paid off: the Peloponnese sided with Antigonus. Cassander was almost defeated and opened negotiations. Before he could conclude a separate peace with Antigonus, however, his allies Lysimachus and Ptolemy renewed their offer of assistance, and Cassander decided to continue the struggle. When Antigonus wanted to invade Europe, Lysimachus was ready for him, and Antigonus' attack came to nothing.
In the meantime, Ptolemy was gathering his forces. Although Tyre had fallen, he still had a large navy and decided to attack Cilicia (summer 312). At Gaza, his army was intercepted by Antigonus' son Demetrius and his officers Peithon of Babylonia and Nearchus, who were unable to overcome the ruler of Egypt (autumn). Ptolemy proceeded to Syria, but when Antigonus arrived, he returned home, knowing that his forces were no match for the lord of Asia (winter 312/311).
At this moment, Antigonus made the greatest mistake of his life. One of the commanders in Ptolemy's army was Seleucus, the former satrap of Babylonia. While the forces of Ptolemy were retreating, he took his units and instead of marching to Egypt, crossed the desert and advanced to Babylon, where he was recognized as satrap on 1 June 311. Although the consequences of this action were, at that time, unclear, Seleucus turned out to be Antigonus' nemesis.
By now, it had become clear that Antigonus and Demetrius could not defeat Ptolemy, Lysimachus, and Cassander in the near future. A peace treaty was concluded in December. Ptolemy and Lysimachus were confirmed in their territories; Cassander and Antigonus remained supreme commanders of the Macedonian forces in Europe and Asia; the Greek towns were recognized by all parties as 'free and autonomous' (but Cassander kept garrisons at several places); and it was agreed that the boy king Alexander, son of Alexander the Great and Roxane, would become sole ruler of the entire empire when he came of age, in 305. The result of the treaty was, of course, that Roxane and the twelve year old Alexander were killed. This was the end of the Macedonian royal house. (The story is told here.)
In a letter to the Greek towns (text), Antigonus explained why he had not continued the war. He did not want it to last and wanted to put an end to the destruction of Greece. This was less hypocritical than it may sound: Antigonus wanted the Greek towns as allies, which was in the realm of the possible, and not as subjects, which was impossible. Another motive was that he had now covered his back, and could attack Seleucus, whom he had by now recognized as a dangerous enemy.