The Hellenistic Period started with the death of Alexander the Great in 323 BC and ended with the annexation of Greece by Rome in 146 BC, although Hellenistic Kingdoms outside of Greece continued until 30 BC with the death of Cleopatra VII.
After the death of Alexander the Great, his generals broke apart his empire to form their own kingdoms and fought a series of wars against each other, known as the Wars of the Diadochi. The most important of these rulers in the decades after Alexander’s death were Ptolemy I in Eygpt and Seleucus I in Syria and the former Persian Empire.
During the Hellenistic Period, the importance of actual Greece within the Greek-speaking world declined rapidly. Instead of Athens, the great capitals of Hellenistic culture were Alexandria in the Ptolemaic Kingdom and Antioch in the Seleucid Empire. The conquests of Alexander the Great greatly widened the horizons of the Greeks as a whole and led to the emigration of many Greeks to the Hellenistic Kingdoms in the East.
In this period, the Greek city-states formed themselves into two leagues; the Achaean League (including Thebes, Corinth and Argos) and the Aetolian League (including Sparta and Athens). The leagues were often at war with each other or fighting with or against the Diadochi.
The Antigonid dynasty of Macedonia came into conflict with the Roman Republic in the late 3rd c. BC and after a series of wars, known as the Macedonian Wars, Macedonia annexed by Rome in 149 BC. The two leagues also came into conflict with Rome at this time and were each defeated in turn. The Roman Republic annexed Greece in 146 BC after the Sack of Cornith.