Book IV opens with yet another revolt from allies of the Athenians, this time the city of Messana. Syracuse encouraged the revolt to prevent Athens from a clear path to Sicily. Additionally, Athens is again invaded by the Spartans under King Agis, son of Archidamus.
Meanwhile an Athenian fleet builds a fort as an outpost in the Peloponnesus (Pylos), under Demosthenes. This sends the forces back to Spartan territory. Demosthenes rallies his troops in defense of their outpost, ironically Athenians defend their occupied Spartan land while the Spartans attack from the sea. Thucydides notes this irony.
The Spartans initiate an armistice, initially framed as a “treaty…to end the war, and offer peace and alliance” (4.19). However, the Athenians are swayed by the demagogue, Cleon a powerful speaker who is popular with the multitudes.
As the war drags on, Cleon is blamed for not accepting the Spartan treaty, but he blames the general, Nicias who promptly resigns. Thus Clean leads an Athenian force with Demosthenes to attack the Spartans. After much fighting, the Spartans take heavy losses and surrender, with Cleon returning to Athens emboldened.
Brasidas successfully encourages revolt in several Athenian provinces, and Pagondus encourages attacks on Athens. Gains and losses are made by both the Spartans and the Athenians. The Athenians retreat from battling the Boetians who commit a great sacrilege by not returning their dead. Thucydides notes his personal part in the story at (4.104) in the battle for Amphipolis, an Athenian colony, which eventually fell to the Spartans and caused great dismay for the Athenians due to its timber resources and geopolitical importance.
In the Spring of 423 BC the Athenians secure a truce, a one year armistice, in order to prevent the loss of any more cities to revolts. Thus concludes Book IV.