Answer:
Religion, language, and cultural background
Step-by-step explanation:
In ancient times, Greece was not a singular country that we know of today, but a group of city-states known as polis. Still, they were connected by common traits.
The main thing connecting them was religion. Each city-state had a focus on its own deity of worshiping and their preferred pantheon, but they all believed in the same gods and myths.
They also shared the language, one of the most important elements of social and cultural identification and grouping. They gathered around the idea of the same heritage and background.
Therefore, in case of foreign attacks and invasions (as was Persian wars), city-states would stick together, based on these ideas, and would protect against the shared enemy.