I won't tackle your point on the origins of the Peloponnesian War, as I am not qualified to provide an in-depth answer. It is best to wait for someone else to answer that.

I will, however, clarify some of the assumptions you made in your question.

Firstly, the Peloponnesian War was not a civil war. The Greeks did not exist in a single unified entity but instead were grouped into more than 1000 poleis, which is commonly translated as 'city-state'. These poleis stretched from mainland Greece into the Black Sea and to southern France. Greeks were everywhere. There were, of course, alliances and leagues that combined these poleis into 'blocs', such as the Peloponnesian League (headed by Sparta) or the Delian League (headed by Athens, at the time of the Peloponnesian War), but membership in these leagues did not automatically mean a polis was no longer independent.

We only have significant evidence for a small handful of these poleis, primarily Sparta and Athens, so it is difficult to make any statement about 'Ancient Greece' that does actually refer to the whole, as it is possible that every polis was slightly different from one another. The differences between the few poleis we do actually know about does suggest that there were some significant differences, such as political organisation. There were monarchies (both kings and tyrants), oligarchies, and democracies).

So, as you can see, Greece was not a 'perfect country'. That isn't even going into many other issues that make ancient Greece far from perfect, such as the reliance on slave labour.

Secondly, ancient Greece was by no means the 'most advanced' civilisation at this time. Greece certainly appears, to us, to be an advanced civilisation. However, this belief stems from several factors that effectively mean we are biased to automatically view the Greeks this way. The most prevalent bias is the belief that the Greeks were the origins of 'Western' civilisation. However, as I'll go into below, the Greeks very much belonged to the cultural sphere of the Near East.

The total surviving evidence makes it seem that the Greeks had a greater intellectual output than other ancient societies, but this is very much an accident. Other societies had similar intellectual outputs, and the Greeks' own culture was largely a product of inheriting different elements, such as art and literature, from Near Eastern societies. The 'foundational' texts of ancient Greek society, Homer and Hesiod, have very strong connections to older eastern literature, suggesting, if not a conscious borrowing, that these writers existed within a cultural sphere centered further east. These eastern societies did not stop these intellectual pursuits after the Greeks gain the limelight in the historical record. However, very little survives from these societies, compared to ancient Greece, that can tell us about these later developments. Ancient Greece was certainly advanced in its own way, developing upon these early ideas in a distinctly Greek way, but there is nothing to suggest they were the 'most advanced'.

Ultimately, our own cultural bias, one born from the Romans' own fascination with the Greeks, is what gives us this sense of advancement concerning the Greeks. Moreover, modern society projects onto the Greeks, wanting to see in them the origins of our own 'advanced' society. However, most people are likely unaware that Greece was itself very much a product of Near Eastern intellectual developments.