Because the Spartans had the superior army, the Athenian leader Pericles employed a strategy that avoided land battles and relied instead on control of the sea. When the war broke out, most Athenians crowded into the city, leaving the outlying areas of Attica open to invasion. Sparta's strategy was to invade yearly, as it did from 431 to 425, except in 429 and 426, hoping to break Athens's will and to encourage Athens's subjects to rebel.
The first stage of the war, called Archidamian from Archidamus, the Spartan king, ended in stalemate in 421 with the Peace of Nicias. Athens had remained firm and had suppressed the dangerous rebellion of Mytilene in 427. It was most damaged by the onset (430) of plague, which removed perhaps a quarter of the Athenian population and caused Pericles' death (429). Athens gained an advantage in the war in 425 by capturing a Spartan force on the island of Sphacteria, but this victory was canceled the next year when the Spartan Brasidas captured Amphipolis. The deaths (422) of Cleon and Brasidas, both of whom were prowar, led to a truce in 421.
The peace was unstable because, although there
were no significant hostilities, neither side fully complied with the terms
of the agreement. In 415, a year after destroying the inoffensive island-state
of Melos, Athens attempted to conquer Syracuse, largely at the urging of
Alcibiades. The expedition ended disastrously in 413, and the debacle enticed
Sparta into fighting once more. This last stage of the war is called "Decelean"
from the name of a town in Attica, Decelea, which Sparta fortified--to
the enormous cost of the Athenians. But the war was won on the sea. Aided
by Persian resources, Sparta became a naval power. It encouraged the rebellion
of Athens's allies; proceeding north from Chios to the Hellespont, Sparta
gradually overcame the Athenian navy in spite of effective countermeasures
taken by Alcibiades and others. Lysander won the decisive battle of Aegospotami
in 405; Athens was blockaded and surrendered (April 404). Athens gave up
its fleet, submitted to the destruction of its fortifications, and suffered
the rule of an oligarchy, the Thirty Tyrants. The imperial city never recovered
from the blow, although the Thirty Tyrants were deposed in 403. Thucydides'
History of the Peloponnesian War is the principal
source for the events of the war up to 411.