Undoubtedly the geography of Greece—divided and sub-divided by hills, mountains and rivers—contributed to the nature of ancient Greece. On the one hand, the ancient Greeks had no doubt that they were 'one people'; they had the same religion, same basic culture, and same language. Yet each city-state or "polis" was independent; unification was something rarely discussed by the ancient Greeks. Even when, during the second Persian invasion of Greece, a group of city-states allied themselves to defend Greece, most "poleis" remained neutral, and after the Persian defeat, the allies quickly returned to infighting.