Diplomacy has failed. The weapons now take center stage. Following the unprovoked invasion of the Thebans in Plataea, which unfolded into a tragedy with three acts, a long war begins with disastrous consequences for Greece.

Before the Peloponnesians invade Attica, Thucydides provides us with the alliances of the Greek cities and the forces of the warring parties. This is followed by hostilities and Pericles' Epitaph, which is a hymn to democracy.

Next, we have a description of the plague that struck Athens, the first reporting on public health issues. This is followed by the confrontation between Athenians and Spartans in the bay of Patras and in Naupactus.