The Aeacidae, a well-respected family of Epirus who are members of the Molossian royal house and claim descent from Achilles, are the only Epirotes regarded by the Greeks as Greek.

From about 370 BCE on, the Aeacidae have been able to expand the Molossian state by incorporating tribes from the rival groups in Epirus.

Their efforts gain impetus from the marriage of Philip II of Macedon to their formidable princess, Myrtale, the daughter of the late King Neoptolemus, who died around 360 BCE, and niece of his brother and successor, Arybbas.

Apparently, the princess—who in the near future will become known as Olympias—was originally named Polyxena, as Plutarch mentions in his work Moralia, and changed her name to Myrtale prior to her marriage to Philip II of Macedon as part of her initiation into an unknown mystery cult.

Philip had first fallen in love with Myrtale when both were initiated into the mysteries of Cabeiri at the Sanctuary of the Great Gods, on the island of Samothrace.

Her younger brother Alexander is sent to Macedonia as well, to receive a Greek education.

Philip invades Molossia in 350 BCE and installs his brother-in-law Alexander as king.

Arybbas flees to Athens (where he will die peacefully in 342 BCE).

Philip's brother-in-law, King Alexander of Molossia (in Epirus, located in present Albania and northwestern Greece, with Illyria to the north, the Pindus mountains to the east, and the Gulf of Ambracia—near Préveza—to the south), around 340 unifies the Epirotes.

Philip will presently lead the grand army into Asia, and the Greeks will be with him.

Perhaps some Macedonian soldiers, who might have preferred Athenian loot to an Athenian alliance, are puzzled about Philip's motives.

Thus, it may be for the benefit of such doubters that Philip has himself depicted in a domestic Macedonian context (he would surely not risks such a thing in Greece) as a “thirteenth Olympian god.”

On the eve of the Persian invasion, however, during the wedding of his daughter, Cleopatra, to his brother-in-law, Alexander of Molossia in June/July 336, the forty-six-year-old Philip is assassinated in mysterious circumstances, perhaps at the instigation of Persian king Darius III.

The assassin, a bodyguard (somatphylax) named Pausanias, is quickly slain, perhaps accidentally, by one Leonnatus; but suspicion immediately falls upon Alexander, never far from Philip's side that day, and Alexander's abused mother Olympias and her political party, those with most to gain from Philip's death.

Alexander, however, is quickly presented to the army as Macedon's new king, and immediately executes two highly placed suspects, the princes of Lyncestis, alleged to be behind Philip's murder, along with all possible rivals and the whole of the opposition faction.

Not many actual rivals have to be eliminated, however, because Alexander's succession is not in serious doubt.

Amyntas is still alive, but there is no reason for Alexander to see him as a threat (in any case, he is probably dead by 335).

Olympias, on her return from Epirus, has Cleopatra and her infant daughter killed.

Ptolemy returns from exile also and joins the King's bodyguard.

King Alexander of Epirus, also known as Alexander Molossus and an uncle of Alexander III of Macedon, had in 334 accepted an invitation from the Tarentines to intervene in southern Italy, where the divided Greek colonies are under threat by the federation of mountain tribes known as the Samnites, formidable warriors who had, in the preceding century, conquered several Greek towns.

Alexander's most important motive (according to Livy) is the warning of an oracle to beware of the water of Acheron and the city of Pandosia; for it is there that the limits of his destiny are fixed.

This inspires him to cross over into Italy all the sooner, that he might be as far as possible from the city of Pandosia in Epirus and the river Acheron, which flows from Molossia into what Livy calls the Infernal Marshes and finally empties itself into the Thesprotian Gulf.

Alexander of Epirus crosses over to Italy in 333 BCE to assist the Tarentines against the Lucanians, Bruttians and Samnites.

The landing of Alexander near Paestum leads the Samnites to make common cause with the Lucanians, but their united forces are defeated by turn in a pitched battle.

Alexander captures the city of Heraclea, (a Greek town that had been taken by the Italian tribes), …

…takes Sipontum (a pirate haven on central Italy's Adriatic coast) from the Apulians, …

…takes Potentia from the Lucanians, and …

…captures Consentia and …