I've been reading about the Peloponnesian War and have read that Sparta felt threatened by Athens ambitions of Democracy (an allusion of Thucydides).

However, Syracuse was Democratic at the time of the failed Syracuse campaign. Syracuse was not an Athenian colleague obviously, but I don't see friendly relations with Democratic states. It seems Athens was primarily concerned with colonies that could contribute either men or resources to Athens. I haven't read of any cities or colonies being founded or acquired where they adopted Democratic ideals.