The Hellenistic period began with the wars of the Diadochi, armed contests among the former generals of Alexander the Great to carve up his empire in Europe, Asia, and North Africa. The wars lasted until 275 BC, witnessing the fall of both the Argead and Antipatrid dynasties of Macedonia in favor of the Antigonid dynasty. The era was also marked by successive wars between the Kingdom of Macedonia and its allies against the Aetolian League, Achaean League, and the city-state of Sparta.
During the reign of Philip V of Macedon (r. 221-179 BC), the Macedonians not only lost the Cretan War (205-200 BC) to an alliance led by Rhodes, but their erstwhile alliance with Hannibal of Carthage also entangled them in the First and Second Macedonian War with ancient Rome. The perceived weakness of Macedonia in the aftermath of these conflicts encouraged Antiochus III the Great of the Seleucid Empire to invade mainland Greece, yet his defeat by the Romans at Thermopylae in 191 BC and Magnesia in 190 BC secured Rome's position as the leading military power in the region. Within roughly two decades after conquering Macedonia in 168 BC and Epirus in 167 BC, the Romans would eventually control the whole of Greece.
During the Hellenistic period the importance of Greece proper within the Greek-speaking world declined sharply. The great centers of Hellenistic culture were Alexandria and Antioch, capitals of Ptolemaic Egypt and Seleucid Syria respectively. Cities such as Pergamon, Ephesus, Rhodes and Seleucia were also important, and increasing urbanization of the Eastern Mediterranean was characteristic of the time.
The quests of Alexander had a number of consequences for the Greek city-states. It greatly widened the horizons of the Greeks, making the endless conflicts between the cities which had marked the 5th and 4th centuries BC seem petty and unimportant. It led to a steady emigration, particularly of the young and ambitious, to the new Greek empires in the east. Many Greeks migrated to Alexandria, Antioch and the many other new Hellenistic cities founded in Alexander's wake, as far away as what are now Afghanistan and Pakistan, where the Greco-Bactrian Kingdom and the Indo-Greek Kingdom survived until the end of the 1st century BC.
The defeat of the Greek cities by Philip and Alexander also taught the Greeks that their city-states could never again be powers in their own right, and that the hegemony of Macedon and its successor states could not be challenged unless the city states united, or at least federated. The Greeks valued their local independence too much to consider actual unification, but they made several attempts to form federations through which they could hope to reassert their independence.