The Spartans immediately tried to destroy the new regime. Other states seem to have been more positive, and many eventually adopted more democratic systems themselves.

It may go without saying, but I just want to stress here that Athenian democracy did not happen because one day a random Athenian citizen sat up in bed and said "You know what we should do? Give the people power." No such goal had been formulated yet. I've written about the rise of Athenian democracy itself in more detail here; in brief, the reforms introduced by Kleisthenes in 508/7 BC were intended to break the power of elite factions struggling for supreme power, not to bring Athens closer to a new ideal of equality and democracy. Furthermore, the specific solutions he came up with fit into a framework of shifting political practice throughout the Greek world. In particular, as far as we can tell, all states already had a general assembly of all male citizens (though usually without much power); Kleisthenes did not introduce this. Like many other states, Athens also already had an executive Council, which allowed a broader segment of the population to take a regular part in politics. Kleisthenes' most far-reaching reforms were essentially about how to fill this Council. He redrew the map of Athenian tribes (administrative districts), which is something that lawgivers and tyrants elsewhere also did on occasion to break up factional loyalties; he mandated that Council seats be filled by sortition rather than election, which was already a well-established feature of government (especially for filling smaller councils of magistrates) in other Greek states.

In other words, the reforms of Kleisthenes may not have appeared so radical to contemporary Greeks. It certainly wouldn't have seemed to them as if a black swan had suddenly joined their white flock. It is not clear whether the distinction between oligarchy and what was initially referred to as isonomia (equality before the law) had been clearly established yet, and certainly the reforms of Kleisthenes still left the Athenian state with many oligarchic elements. Most likely, the Greeks of the time would have thought of the Athenians' new system as a few steps further in the direction of isonomia than any of them had taken to date, but not so many that the resulting government system was a wholly new thing. It took the Athenians several more decades of reforms and a transformation of their role and status in the Greek world to arrive at what later commentators have called "radical democracy" - the system that we typically mean when we refer to Athenian democracy.

For the Spartans, however, the problem was not the new system or the ideology of equality that soon developed to support it. The problem was simply that they had invested a significant effort to cast out the tyrant Hippias, who had ruled Athens from 527-510 BC, and had intended for the government filling the vaccuum after the removal of Hippias to be reliably pro-Spartan. During the Spartan campaigns in Athenian territory, the Spartan king Kleomenes became close friends with the Athenian Isagoras, and Kleomenes seems to have increasingly felt that this Isagoras was the right man to rule the Athenians. Isagoras therefore became the most prominent rival of the already well-established Kleisthenes in a renewed political struggle for supreme control of the city. When Kleisthenes used the Assembly to push through his reforms, fragmenting the local base of support that Isagoras was drawing on, Isagoras promptly called for Spartan aid, exiled Kleisthenes and his most prominent allies, and attempted to dissolve the Council. The chronology is confused and our sources are not in exact alignment, but this seems to be the order of events: Kleisthenes tried to establish a more democratic system, at which point Isagoras tried to seize power. While we don't know what sort of system Isagoras intended to establish, the options are either a narrow oligarchy of him and his 300 wealthy friends, or a fresh tyranny propped up by Kleomenes and the Spartans.

Unfortunately for Isagoras and Kleomenes, their plans for Athens were popular with no one. On the Athenian side, the Council flatly refused to be disbanded, and Isagoras, stopping short of massacring those he meant to rule, felt he had no choice but to flee to the Akropolis. According to our sources, the people then rose up en masse to besiege Isagoras on the Akropolis, and he soon surrendered. Many scholars have taken the actions of the Council and the people as a sign that Kleisthenes' new system had the overwhelming and passionate support of the Athenian citizen body. A more cynical commentator might simply say that they were sick of elite in-fighting and would do anything to avoid another bout of tyranny.

As for Kleomenes, when Isagoras called for help, he immediately raised the army of the Peloponnesian League, arranged for a joint strike with his allies north of Athens, and marched toward Athens. However, when the army arrived at the border of Athenian territory, the Corinthians in the army refused to march on; many other subject allies soon followed suit. Eventually even the other Spartan king, Demaratos, sided with the mutineers and decided to lead the army home. Kleomenes was left with no forces to impose his will on the Athenians, and was forced to watch as the Athenians drove Isagoras into exile and defeated the alliance of Boiotians and Chalkidians that had agreed to attack Athens from the north while Kleomenes advanced from the south.

The reason we're given for the mutiny was that the Spartan allies felt that the whole campaign was just Kleomenes' private project, and that their lives were being put at risk for his unjust desire to put a personal friend in charge of the Athenians. However, we may suspect that there was an element of sympathy, too, for the new Athenian system of government, which was emphatically the Athenians' own choice. Many cities of the Peloponnesian League suffered the establishment of pro-Spartan oligarchic regimes, which later became one of the key distinctions between Athenian and Spartan hegemony. Noting the march of popular institutions of government elsewhere, the Peloponnesian League states may well have sided with Athens and against Sparta in the question of whether Kleisthenes' reforms were allowed to persist.

Whatever the case may be for the states of the Peloponnese, it is certain that the Athenian experiment of a widened franchise and greater equality proved to have a wide appeal, and many cities elsewhere soon appear to have entertained thoughts of adopting it themselves. We know that when the Ionian cities revolted against Persia in 499 BC, they all ousted their Persian-backed tyrants and installed democratic systems of government. This would have included the Ionian islands as well as cities in Aiolia and Karia. The development is really quite spectacular; apparently, in just 8 years, the Athenian system had become so popular that these communities were willing to risk the wrath of Persia to have some form of it for themselves. It is possible that the institutional changes required were not very great - the opening up of existing councils, the transfer of powers to the Assembly - but it nevertheless represented the firm establishment of a new system within the range of Greek political thought and practice.

There was never a linear spread of democracy throughout the Greek world; many states went back and forth between democracy and oligarchy, influenced by expedience as much as ideology, and many never adopted democracy at all. Nevertheless, it seems fair to say that many Greeks from the early Classical period onwards thought the Athenian idea had something in it, especially if they yearned for the same level of autonomy that the Athenian people had acquired for itself.