Several times throughout its existence, city-states with membership in the Delian League, due to profound dissatisfactions over a vast array of subjects, deflagrated insurgencies, mainly to free themselves against the abusive, sometimes dictatorial – even tyrannical – domain of Athens, being mostly defeated in the vast majority of occasions. Athens wisely used these wars at their favor, with another purpose in mind: to intimidate and threat other cities with potential to stage a rebellion. Since Athens usually defeated the insurgents in bloody and humiliating battles, other city-states with membership in the Delian League would refrain from taking the same path. The so called "golden period”, also referenced as the classic history of Athens, came to an end when the city was devastated after the Peloponnesian War.
Although it is entirely possible to understand the concept of city-state, Ancient Greece saw, indeed, periods of political, military and ideological unification. The fact that each and every city-state detained a vast degree of autonomy does not imply an intrinsic factor of complete independence: for the greater part of its existence, the Delian League was led by Athens, assuming the same role Sparta did, concerning the Peloponnesian League (and both, by the way – just for the record – often did it with iron hand, never tolerating rebellion or disobedience of any kind).
So, to affirm that Ancient Greece was a disjointed and disconnected ordeal of city-states is an entirely wrong supposition. Although it has never been a cohesive union, history teaches us that there were fairly organized confederations of city-states, united by a strong leadership – that at certain times acted in an excessively dictatorial mode, its true – comprising a coherent unity, fighting for common interests. The Delian League and The Peloponnesian League, guided by the strong leadership of Athens and Sparta respectively, were two city-states that divided Ancient Greece into two belligerent, distinct and rival poles, often exerting with iron fist the political and ideological ambitions which both represented and fought for, obviously moved by the ardent desire to guarantee their political hegemony. Something that nation-states to this day try to enforce by the excruciating, tragic, inhumane and violent means of war.