When Alexander the Great died at Babylon in 323 BC, his mother Olympias immediately accused Antipater and his faction of poisoning him, although there is no evidence to confirm this. With no official heir apparent, the Macedonian military command split, with one side proclaiming Alexander's half-brother Philip III Arrhidaeus (r. 323 – 317 BC) as king and the other siding with the infant son of Alexander and Roxana, Alexander IV (r. 323 – 309 BC). Except for the Euboeans and Boeotians, the Greeks also immediately rose up in a rebellion against Antipater known as the Lamian War (323–322 BC). When Antipater was defeated at the 323 BC Battle of Thermopylae, he fled to Lamia where he was besieged by the Athenian commander Leosthenes. A Macedonian army led by Leonnatus rescued Antipater by lifting the siege. Antipater defeated the rebellion, yet his death in 319 BC left a power vacuum wherein the two proclaimed kings of Macedonia became pawns in a power struggle between the diadochi, the former generals of Alexander's army.A council of the army convened in Babylon immediately after Alexander's death, naming Philip III as king and the chiliarch Perdiccas as his regent. Antipater, Antigonus Monophthalmus, Craterus, and Ptolemy formed a coalition against Perdiccas in a civil war initiated by Ptolemy's seizure of the hearse of Alexander the Great. Perdiccas was assassinated in 321 BC by his own officers during a failed campaign in Egypt against Ptolemy, where his march along the Nile River resulted in the drowning of 2,000 of his men. Although Eumenes of Cardia managed to kill Craterus in battle, this had little to no effect on the outcome of the 321 BC Partition of Triparadisus in Syria where the victorious coalition settled the issue of a new regency and territorial rights. Antipater was appointed as regent over the two kings. Before Antipater died in 319 BC, he named the staunch Argead loyalist Polyperchon as his successor, passing over his own son Cassander and ignoring the right of the king to choose a new regent (since Philip III was considered mentally unstable), in effect bypassing the council of the army as well.Forming an alliance with Ptolemy, Antigonus, and Lysimachus, Cassander had his officer Nicanor capture the Munichia fortress of Athens' port town Piraeus in defiance of Polyperchon's decree that Greek cities should be free of Macedonian garrisons, sparking the Second War of the Diadochi (319–315 BC). Given a string of military failures by Polyperchon, in 317 BC, Philip III, by way of his politically engaged wife Eurydice II of Macedon, officially replaced him as regent with Cassander. Afterwards, Polyperchon desperately sought the aid of Olympias in Epirus. A joint force of Epirotes, Aetolians, and Polyperchon's troops invaded Macedonia and forced the surrender of Philip III and Eurydice's army, allowing Olympias to execute the king and force his queen to commit suicide. Olympias then had Nicanor and dozens of other Macedonian nobles killed, but by the spring of 316 BC, Cassander had defeated her forces, captured her, and placed her on trial for murder before sentencing her to death.Cassander married Philip II's daughter Thessalonike and briefly extended Macedonian control into Illyria as far as Epidamnos. By 313 BC, it was retaken by the Illyrian king Glaucias of Taulantii. By 316 BC, Antigonus had taken the territory of Eumenes and managed to eject Seleucus Nicator from his Babylonian satrapy, leading Cassander, Ptolemy, and Lysimachus to issue a joint ultimatum to Antigonus in 315 BC for him to surrender various territories in Asia. Antigonus promptly allied with Polyperchon, now based in Corinth, and issued an ultimatum of his own to Cassander, charging him with murder for executing Olympias and demanding that he hand over the royal family, King Alexander IV and the queen mother Roxana. The conflict that followed lasted until the winter of 312/311 BC, when a new peace settlement recognized Cassander as general of Europe, Antigonus as "first in Asia", Ptolemy as general of Egypt, and Lysimachus as general of Thrace. Cassander had Alexander IV and Roxana put to death in the winter of 311/310 BC, and by 306–305 BC, the diadochi were declared kings of their respective territories.

Question: Who did Philip III replace Polyperchon with as regent?
Answer: Cassander