Philip Arrhidaeus (c.356-317): the mentally deficient and epileptic brother of Alexander the Great who succeeded him as king of the Macedonian Empire in 323, but had several regents, who all used their pupil for their own purposes.
Arrhidaeus was the son of the Macedonian king Philip II (360-336) and a Thessalian woman named Philinna. He was of about the same age as Alexander; however, Alexander's mother was Philip's lawful wife Olympias, and therefore, Alexander was recognized as the crown prince. His position was secure, but he did not really feel that way, and when the satrap of Caria, Pixodarus, wanted to engage his daughter to Arrhidaeus, Alexander intervened (c.337?).
During Alexander's reign (336-323), Arrhidaeus was more or less isolated. It is not even known where he was staying during his brother's campaigns, although it is reasonable to assume that he stayed behind in Macedonia, where Antipater, the supreme commander of the Macedonian forces in Europe, would oversee the behavior of the mentally unstable and epileptic prince.
Wherever he may have been during the reign of Alexander, he certainly was at Babylon when the king died on 11 June. Next day, the Macedonian generals met to discuss the new situation. Under normal circumstances, they, as representatives of the Macedonian nation in arms, had to choose a new king, and the most likely candidate would be prince Arrhidaeus. However, he was illegitimate and mentally unfit to rule. As a consequence, it was difficult to reach a solution.
Perdiccas, the commander of the Companion cavalry who had been appointed by Alexander as his successor, said that it was best to wait until queen Roxane, a Sogdian princess by descent who was now pregnant, had given birth. If it were a son, it would be logical to make him king. This was all too transparent: Perdiccas wanted to be in sole command until the boy had grown up - at least eighteen years.
On the other hand, the commander of the phalanx, Meleager, said that Arrhidaeus was the closest relative of Alexander and should therefore become king. The infantry supported this proposal, because Arrhidaeus was of Macedonian blood - as Roxane's son could never be. Another reason for the soldier's choice may have been that they wanted the empire to be a unity, whereas Perdiccas and the other cavalry commanders seemed to be aiming at a division of the kingdom (text).
The situation was tense, as it seemed that Meleager's soldiers wanted to fight for Arrhidaeus against Perdiccas and his adherents. That would mean a war between infantry and cavalry. Although violence was used and Meleager was killed, the cooler heads on both sides improvised a compromise. Perdiccas was to be regent for king Arrhidaeus and Roxane's son (if the baby were a son, of course). Seeing that this was the only way to prevent civil war, everybody agreed. Arrhidaeus became king under the throne name of Philip III, Roxane's baby turned out to be a son (Alexander).
At about the same time, he was married to a noblewoman named Eurydice. Our sources are extremely hostile to the queen, but it seems that she sincerely wanted to protect her husband from being used by his regents, and incurred -as a consequence- their everlasting hatred.
Philip Arrhidaeus was now king, but Perdiccas was the ruler. He issued his own orders under the name of king Philip. This could have worked, but Perdiccas became too powerful, especially when Olympias offered her daughter Cleopatra to the regent. This would make him the brother-in-law of Alexander the Great, and a more direct heir to the throne than Philip Arrhidaeus, who was, after all, a bastard.
Civil war (the First Diadoch War) broke out in the last months of 322: Perdiccas was attacked from several sides by the satrap of Egypt Ptolemy, the generals Craterus and Antigonus Monophthalmus, and Antipater, still the supreme commander of the Macedonian forces in Europe. A year and a half later, Perdiccas was murdered by his own officers, and a new settlement was necessary (320). This time, the royal family - king Philip, the baby Alexander, Roxane - was placed under the regency of Antipater, and moved to Europe. Eurydice saw that her husband had became a pawn in a game, but her attempts to prevent this were in vain. From now on, Philip Arrhidaeus was to do what Antipater wanted.
When the new regent reorganized the monarchy (text), he had conspicuously ignored queen Eurydice, who was angry. She did not have to wait very long to get a second chance: in the autumn of 319, Antipater succumbed to old age. He had appointed the old officer Polyperchon as his successor, but Antipater's son Cassander felt ignored, and revolted, supported by Eurydice. Almost immediately, he received the support of Antigonus, who saw a chance to increase his power. This was the beginning of the Second Diadoch War.
However, Polyperchon found an ally too. Antigonus was the supreme commander of the Macedonian forces in Asia, but Philip Arrhidaeus could, of course, appoint another man in this office. It is not clear how Antipater overcame the opposition by Eurydice, but it worked: king Philip (i.e., Polyperchon) appointed Eumenes as new supreme commander in Asia. This man had earlier fought for Perdiccas and Philip, and now fought for Polyperchon and Philip. Antigonus was occupied with this war until 315.
In the meantime, Cassander and Eurydice had expelled Polyperchon and the other members of the royal family (Roxane, the boy king Alexander, Olympias). In the spring of 317, Antipater's son was recognized as ruler of Macedonia and regent of king Philip Arrhidaeus. Cassander now advanced to the south, to subdue the towns of the Peloponnese. Immediately, Olympias and king Aeacidas of Epirus invaded Macedonia. It was not a very powerful coalition, but they could play one trump card: the boy Alexander was the lawful successor of the great Alexander, whereas Philip Arrhidaeus was a mere bastard and mentally unstable.
Philip Arrhidaeus and Eurydice met them at the frontier - Cassander was still campaigning in the Peloponnese - but their entire army deserted them and joined the enemy (text). Olympias ordered the execution of her stepson Arrhidaeus and forced Eurydice to commit suicide (25 December 317, according to a recently identified fragment of the Astronomical Diaries).