These don't necessarily  have to be major event but for the purposes of interesting storytelling they usually are.

    For most of this time the Greeks did not have a single government or ruler. There were a number of city states, each with its own constitution. Athens, Sparta and Corinth are examples. Some had kings, and some, like Athens, had a form of democracy. As time went on, the most powerful cities collected other cities into confederations known as "leagues". This applied to many of the Greek colonies in Asia Minor, most of which had close ties to one or other of the big three cities.

    In the middle of this period was classical Greece, which flourished during the 5th to 4th centuries BC. Athenian leadership successfully repelled the threat of Persian invasion in the Greco-Persian Wars. The Athenian golden age ends with the defeat of Athens at the hands of Sparta in the Peloponnesian War in 404 BC.
    In the last, Hellenistic, period Greece was unified by the conquests of Alexander the Great. The city states continued, under the overall influence of Macedonia.
    Greek culture had a powerful influence on the Roman Empire, which carried a version of it to many parts of the Mediterranean region and Europe. In this way, classical Greece was part of the foundation of Western civilization. Greek was also the language, and partly the culture, of the Byzantine Empire." - Wikipedia