In one night, most of the stone Hermes figures at Athens' doorways have their faces mutilated, just as the fleet for Sicily is being prepared.
In the midst of these preparations all the stone Hermae in the city of Athens, that is to say the customary square figures so common in the doorways of private houses and temples, had in one night most of them their faces mutilated.
Athenians took it as a bad omen for the expedition and as evidence of a plot to overthrow the government. Alcibiades was later put under arrest, partly over the herms.
Based on Wikipedia: Sicilian Expedition
This event was taken very seriously by the Athenian people as it was considered a bad omen for the expedition, as well as evidence of a revolutionary conspiracy to overthrow the government.
The fleet proceeded to Catania, where an Athenian ship arrived to inform Alcibiades that he was under arrest, not only for the destruction of the hermai, but also for supposedly profaning the Eleusinian Mysteries.