The Digression on Tyranny begins just after the Salaminia arrives for Alcibiades.

Its purpose is to explain the atmosphere in which the investigation of the mutilation of the herms took place.

The digression is introduced in 6.53.1, with an observation about Athenian traditions concerning the sixth-century tyranny of the Peisistratids....

The people knew from hearsay that Peisistratos' tyranny and that of his sons had become harsh towards the end and, what was worse, that its overthrow had not been due to themselves or Harmodios but the Lacedaimonians. So they remained in a constant state of apprehension and looked with suspicion upon everything.

...and concludes in 6.60.1 with a discussion about the effects of this tradition on the political atmosphere of the late fifth century. This is where section 18 begins.

In addition, as related in 6.61.2, the presence at the same time of a small Spartan force in the Isthmus was misinterpreted as a presursor to an assault on Athens.

For about the same time as the uproar about the Herms, a small force of Lacedaimonians advanced as far as the Isthmus, in pursuance of some scheme with the Boiotians. But the people thought that the Spartans had arrived by agreement with Alciiades and at his instigation. And they believed that, if they had not forestalled the conspirators by arresting them on the basis of the denunciations, the city would have been betrayed. For one night they even slept armed in the temple of Theseus within the walls.