For 150 years before Alexander's birth, the city-states of mainland Greece fought off-and-on with each other and with foreigners. At the time, towns along the eastern Aegean Sea (known then as Asia Minor and today as Turkey) were also Greek.
Those Greek Asia Minor cities (like Ephesus) were frequently dominated by non-Greek powers. They once owed their allegiance to the Kingdom of Lydia (anchored in its capital of Sardis where a temple to Artemis [Diana] was fourth largest in the ancient world). Thereafter, they were run by Persia (today's Iran) after Lydia was overwhelmed by the meteoric rise of Persia as an imperial power.
In 499 B.C., when Greek cities in Asia Minor rebelled against Persian control, forces from the Greek mainland came to their aid. Darius (in 490) and Xerxes (in 480 B.C.), retaliating against the helpers, launched two unsuccessful invasions.
Persian defeats meant the mainland was temporarily secure from foreigners, but prominent Greek cities soon relapsed into hostilities between themselves. The long years of fighting between Sparta and Athens (known as the Peloponnesian War) exhausted everyone.
Shifting loyalties continued and, by 386 B.C., Athens and Sparta conceded Persian dominion over the Greek cities of mainland Asia Minor in exchange for recognition of their own claims. That peace also was short-lived, with endless acts of violence and destruction continuing even when patriotic motives were not obvious.
Macedonia had been largely exempt from this miserable state of affairs. Spared from Greco-Persian political wrangling, Macedonia was not part of the treaty which forfeited the Greek cities of Asia Minor. So when Philip II, who had been acting as regent in Macedon, made himself king in 358 B.C., he and his highly trained standing army ultimately accomplished what the Greeks needed but could not achieve. Philip imposed peace.
Summoning the Greek city-states to a conference in Corinth, Philip emerged as leader of a Greek federation which would wage an offensive against Persia. Before he could enact his plans of war, however, Philip was murdered in 336.
Executing his father's assassin without asking questions, and having anyone else who threatened his position as Philip's heir killed as well, Alexander - at age 20 - became king of Macedonia.
Anyone who thought he was not up to the job would soon learn otherwise.