The conservative general Nicias warned that Athens' current empire was still rebellious, and that any attempt to conquer Sicily ran the risks of imperial overreach. Unsurprisingly, Alcibiades was the most eager to go to war. (It is at this point that Thucydides comments that the Athenians' unwillingness to trust Alcibiades despite his competence as a commander was the city's undoing). This meant siding with Sicily's non-Greek inhabitants against fellow Greeks, which apparently made many people uneasy, because he felt the need to defend it. Alcibiades was so persuasive that Nicias saw his position as hopeless and tried to persuade the Athenians to drop the project by giving an exaggerated estimate of the forces needed. Like some many attempts at "11 dimensional chess," this one backfired and mostly succeeded in convincing the Athenians that so large a force would be invincible. They proceeded to raise it, both in Athens and from allies.
Plutarch reports a great many unfavorable omens, while Thucydides says that the people believed any number of "soothsayers and prophets" who promised success. Most likely there were any number of contradictory omens, good and bad. People paid attention to the good omens ahead of the expedition and the bad ones in retrospect. But the one alarming event that no one could ignore was the mutilation of the herms.
So what can we say with any confidence about this crime? First of all, it was not the figment of anyone's imagination; it actually happened. The mutilated statues throughout the city bore witness to it. Second, it was not a random act or a drunken party that got badly out of hand. Mutilation of all the statutes throughout the city would call for a conspiracy large enough and with enough advance planning to vandalize a considerable number of statues over a considerable area. Third, whoever did it was almost certainly an opponent of the expedition. At best, they were trying to scare Athens into calling it off. At worst, they may have been trying to bring misfortune upon it. Fourth, there were rumors that Corinthians may have done it to block the expedition because Syracuse was a Corinthian colony. And certainly it is possible that foreigners may have been involved. But it seems most likely that the operational end was carried out by people who knew their way around Athens in the dark and knew exactly were all the statues were located, in other words, by citizens or at least permanent residents. Fifth and most controversially, it seems most likely that the offenders were someone from aristocratic, intellectual, or philosophical circles who had become irreligious and therefore did not fear the wrath of the gods for defacing their images! Finally, we should be able to rule out at least one suspect. Alcibiades, as prime mover behind the expedition, is that last person one would expect to do anything to undermine it.
*So how were they vandalized? Thucydides says their "faces" were smashed. My mother, reading this, was rather disappointed, since it was always her understanding that the phalluses were knocked off. An admirer of Thucydides, she was quite confident that he would never use a euphemism. "The translator might," my father said. Well, researching for this blog, I finally got the answer from a modern historian. Thucydides does, indeed, say faces. As for reports that it was the phalluses, this comes from Aristophanes' Lysistrata, in which the sex-starved men show off how stiff and swollen their penises are -- until the chorus leader tells them to cover up before whoever mutilated the herms sees them! So, either Thucydides used a euphemism, or Aristophanes was not going to let accuracy stand in the way of a dirty joke. Take your pick.
**Not one of the first actual moral panics. Presumably the phenomenon itself is much older. But this may be the earliest description of one, although I stand willing to be corrected.