Having pointed out the significance of the war, not only to Greece but to the whole of the world, he gives its causes. To him the real root of the trouble was Sparta's fear of Athenian power: the alleged pretexts were different. The rise of Athens is rapidly described, her building of the walls broken down by the Persians, her control of the island-states in a Delian league which eventually became the nucleus of her Empire, her alliance with Megara, a buffer-state between herself and Corinth. This last saved her from fears of a land invasion; when she built for Megara long walls to the sea she incurred the intense anger of Corinth which smouldered for years and at last caused the Peloponnesian conflagration. The reduction of Aegina in 451 compensated for the loss of Boeotia and Egypt. Eventually the Thirty Years' Peace was concluded in 445; Athens gave up Megara, but retained Euboea; her definite policy for the future was concentration on a maritime empire; she controlled nearly all the islands of the Aegean and was mistress of the Saronic gulf, Aegina, "the eyesore of the Peiraeus", having fallen.

But if she was to confine her energies to the sea, it was essential that she should be mistress of all the trade-routes which in ancient history usually ran along the coast. On both east and west she found Corinth in possession; a couple of quarrels with this city ruptured the peace. In the west, Corinth had founded Corcyra (Corfu); this daughter colony quarrelled with her mother and prevailed. In itself Corcyra was of little importance in purely Greek politics, but it happened to possess a large navy and commanded the trade-route to Sicily, whence came the corn supply. When threatened with vengeance by Corinth, she appealed to Athens, where ambassadors from Corinth also appeared. Their arguments are stated in the speeches which are so characteristic of Thucydides. The Athenians after careful consideration decided to conclude a defensive alliance with Corcyra, for they dreaded the acquisition of her navy by Corinth. But circumstances turned this into an offensive alliance, for Corinth attacked and would have won a complete victory at sea but for timely Athenian succour. In the east Athens was even more vitally concerned in trade with the Hellespont, through which her own corn passed. On this route was the powerful Corinthian city Potidaea, situate on the western prong of Chalcidice. It had joined the Athenian confederacy but had secured independence by building strong walls. When the Athenians demanded their destruction and hostages as a guarantee, the town revolted and appealed to the mother-city Corinth. A long and costly siege drained Athens of much revenue and distracted her attention; but worst of all was the final estrangement of the great trade rival whom she had thwarted in Greece itself by occupying Megara, in the west by joining Corcyra, and in the east by attacking Potidsea.

The final and open pretext for war was the exclusion of Megara from all Athenian markets; this step meant the extinction of the town as a trading-centre and was a definite set-back to the economic development of the Peloponnese, of which Corinth and Megara were the natural avenues to northern Greece. The cup was full; Athenian ambition had run its course. The aggrieved states of the Peloponnese were invited to put their case at Sparta; Corinth drew a famous picture of the Athenian character, its restlessness, energy, adaptability and inventiveness. "In the face of such a rival," they added,

"Sparta hesitates; in comparison Spartan methods are antiquated,
but modern principles cannot help prevailing; in a stagnant state
conservative institutions are the best, but when men are faced with
various difficulties great ingenuity is essential; for that reason
Athens through her wide experience has made more innovations."

An Athenian reply failed to convince the allies of her innocence; one of the Spartan Ephors forced the congress to declare that Athens had violated the peace. A second assembly was summoned, at which the Corinthians in an estimate of the Athenian power gave reasons for believing it would eventually be reduced. They further appealed to what has never yet failed to decide in favour of war—race antagonism; the Athenians and her subjects were Ionians, whereas the Peloponnesians were mainly Dorians. The necessary vote for opening hostilities was secured; but first an ultimatum was presented. If Athens desired peace she must rescind the exclusion acts aimed at Megara. At the debate in the Athenian assembly Pericles, the virtual ruler, gave his reason for believing Athens would win; he urged a demand for the withdrawal of Spartan Alien Acts aimed at Athens and her allies and offered arbitration on the alleged grievances.

It is well to repeat the causes of this war: trade rivalry, naval competition, race animosity and desire for predominance. Till these are removed it is useless to expect permanent peace in spite of Leagues or Tribunals or Arbitration Courts. Further, it should be noted that Thucydides takes the utmost care to point out the excellent reasons the most enlightened statesmen had for arriving at contradictory conclusions; the event proved them all wrong without exception. The future had in store at least two events which no human foresight could discover, and these proved the deciding factors in the conflict.

The war began in 431 by a Theban attack on Plataea, the little town just over the Attic frontier which had been allied with Athens for nearly a century and protected her against invasion from the north. This city had long been hated by Thebes as a deserter from her own league; it alone of Boeotian towns had not joined the Persians. Burning with the desire to capture it, a body of Thebans entered the place by night, seizing the chief positions. But in the morning their scanty numbers were apparent; recovering from panic the Plataeans overwhelmed the invaders and massacred them. This open violation of the treaty kindled the war-spirit. Both sides armed, Sparta being more popular as pretending to free Greece from a tyrant. Their last ambassador on leaving Athenian territory said: "This day will be the beginning of mighty woes for Greece".

The Spartans invaded Attica, cutting down the fruit trees and forcing the country folk into the city; the Athenians replied by ravaging parts of Peloponnese and Megara. The funeral of the first Athenian victims of the war was the occasion of a remarkable speech. Pericles in delivering it expounds the Athenian ideal of life.

"We do not compete with other constitutions, we are rather a pattern
for the rest. In our democracy all are equal before the law; each man
is promoted to public office not by favour but by merit, according as
he can do the State some service. We love beauty in its simplicity, we
love knowledge without losing manliness. Our citizens can administer
affairs both private and public; our working classes have an adequate
knowledge of politics. To us the most fatal error is the lack of
theoretical instruction before we attempt any duty. In a word, I say
that Athens is an education for all Greece; individually we can prove
ourselves competent to face the most varied forms of human activity
with the maximum of grace and adaptability.... We have forced the
whole sea and every land to open to our enterprise. Look daily at the
material power of the city and love her passionately. Her glory was
won by men who did their duty and sacrificed themselves for her. The
whole world is the sepulchre of famous men; their memory is not only
inscribed on pillars in their own country, it lives unwritten in the
hearts of men in alien lands."

At the beginning of the next year a calamity which no statesman could have foreseen overtook Athens. A mysterious plague of the greatest malignity scourged the city, the mortality being multiplied among the crowds of refugees. The city's strength was seriously impaired, public and private morality were undermined, inasmuch as none knew how long he had to live. Discouraged by it and by the invasions the Athenians sent a fruitless embassy to Sparta and tinned in fury on Pericles. He made a splendid defence of his policy and gave them heart to continue the struggle; he pointed out that it was better to lose their property and save the State than save their property and lose the State; their fleet opened to them the world of waters over which they could range as absolute masters. Soon afterwards he died, surviving the opening of the war only two years and a half; his character and abilities received due acknowledgment from Thucydides.