Book V opens at the conclusion of the truce between Athens and the Spartans. Cleon leads the Athenians in an attack on Thrace. A double surprise attack is launched against Cleon and the Athenians by Brasidas of of the Spartans, catching Cleon off guard and kills him en route, as well as Brasidas.
After this cataclysmic juncture, both sides desire peace. Athens feared further revolt from its allies. Spartan men and their land/economy were suffering. New leadership (King Pleistoanax in Sparta, and Nicias in Athens) desired peace. Thucydides called Nicias the “most fortunate general of his time” (5.16). Many of their allies did not agree with the yearning for peace, nevertheless they make a treaty, allying Sparta and Athens for fifty years. Thus ends the “first war” spanning ten years.
Thucydides digresses to discuss the nature of the peace being a mere interval in the ongoing hostilities, rather than a true peace. He mentions his own age being old enough to reflect on the activities and his exile for twenty years after his command of Amphipolous as mentioned earlier. As a result of his exile the Pelopponnesians he was able to see things with far more clarity.
Hostilities resume in the war following the fifty years truce. Alcibiades leads a faction of Athenian allies in diplomacy against Sparta. The Spartans are barred from the Olympic games. The Spartans battle the Argives, allies of the Athenians. The Melians choose to remain allied with the Spartans, despite Athenian warnings of ruin. Eventually the Melians are defeated: the men all killed, the women and children enslaved. Athens settles their country.
For this reading I used the impeccable Landmark edition by businessman-turned classical scholar Robert B. Strassler.