After the defeat of the Persians at Salamis and Plataea in 480 and 479 b.c., Athens rose to the top in Greek politics. As the leading naval power she made herself the strongest member of the Delian League, a grouping of Greek polises (city-states) dedicated to continuing the war against Persia, offensively and defensively. Athens reached the point where she could demand virtually any dues to the League and did not have to account for the money sent to the League treasury. The Athenians spent much of that money turning their city into a cultural and architectural showplace, which did not please the polises contributing the money.

Athens entered into a shifting set of alliances, some voluntary and some forced on weaker neighbors. Between 460 and 445, Athens challenged Sparta and her allies in a number of battles, at the same time providing aid to Egyptians rebelling against Persia. With no clear winner, Sparta and Athens agreed to a truce in 445, but their political rivalry continued. It is difficult to find a more convoluted set of circumstances than the ones that set off the Peloponnesian War. Athenian domination over the northern Greek polises kept them resentful, yet more diplomatic maneuvering and alliance-making resulted in Athens being poised to establish a Greek Empire. The Athenian navy enforced as well as expanded the city-state's dominion over as much of the Mediterranean as possible. Finally, most Greek polises turned to Sparta for leadership. When Athens attempted to meddle in the political affairs of the Peloponnese, long Sparta's sphere of influence, fighting resumed in 431.

The war has been described as the struggle between the elephant and the whale. Sparta had the finest army of the day and Athens possessed the greatest navy, but neither could come to grips with the other. Sparta and her allies besieged Athens after capturing the surrounding countryside, but because they were unable to capture the port of Piraeus, Athens brought in a constant supply of food and other necessities. The Athenian navy also conducted raids on the Spartan coastline, liberating the helots who toiled under Spartan rule. Both powers hurt the other, but neither could finish the other off. Fighting was fairly constant for ten years, until another peace was negotiated in 421. The two main adversaries were glad of the break, but some of their allies were unhappy the peace treaty had not restored lands they had lost in the war. Sparta's and Athens' client states argued and fought among themselves and, when pressed, called on the two powers for aid. Peace was kept for six years, but much of the blame for the renewal of war between them in 414 can be laid at the feet of Alcibiades, a rabble-rousing Athenian general.

Alcibiades convinced the citizenry of Athens to go to war against Syracuse on the island of Sicily. They were providing valuable foodstuffs to Sparta, he claimed. If Athens could establish herself in Sicily (and the Syracusans were originally Greek colonists), then she would be in a commanding position. She would cut off the needed food to Sparta, weakening her to the point of subjection, and possession of Syracuse would allow Athens to dominate the western Mediterranean as well. Defeating the nascent powers of Carthage and Rome, Athens could draw on the manpower of Italy, North Africa, and Iberia (homeland of most of Carthage's mercenary army) to defeat Sparta. Athens could be ruler of the known Western world. Alcibiades' speaking ability won over the citizens to his plan, over the more cautious suggestions of his political rivals. Athens voted to attack. Better still, the Sicilian city of Segesta, currently threatened by Syracuse, offered to pay for the expedition. Alcibiades went to Sicily, with Nicias and Lamachus as co-commanders.

The Athenian invasion force departed in June 415, numbering 134 triremes and 130 transports, the latter carrying about 5,000 hoplites (heavy infantry), 1,300 archers, various javelin throwers and slingers, and 30 horses (about 27,000 troops); 130 supply ships sailed with them as well. The Syracusans ignored rumors of a Greek invasion until the enemy arrived. The only thing that saved them was the standard Greek inability to agree on anything. Nicias wanted to go home once they learned Segesta could not pay for the mission. Lamachus wanted to launch an immediate attack on Syracuse, taking advantage of their lack of preparation and low morale. Alcibiades' oratory again won out. He argued that the Athenians should traverse the island, gathering support from the cities that disliked Syracuse. That, however, gave the Syracusans time to begin training their troops and repairing their defenses. Alcibiades' plan failed, for no Sicilian city voluntarily joined them.

Word soon arrived from Athens that Alcibiades had been convicted in his absence of blasphemy. He was taken away to face judgment but managed to escape his jailers when the ship stopped in Italy. He fled to his former enemy, Sparta, where he revealed Athens' plan for empire and the effect the loss of Syracuse would have on Sparta. At first the Spartans refused, still observing their truce with Athens, but in the end promised to send a general to

command the Syracusan army and a fleet from Sparta's ally, Corinth.

Meanwhile, the Athenians began to attack Syracuse. They defeated a poorly prepared Syracusan force but failed to win a complete victory. Worse, Lamachus was killed in the fighting, leaving the indecisive Nicias in sole command. As winter approached they settled into siege warfare, building a wall parallel to the city's defensive wall, which would cut the city off from any landward help, while the superior Athenian fleet would keep the seaward side covered. Both measures failed. The Syracusans raided the Athenians just often and successfully enough to keep their wall from being finished. Still, morale in the city slipped and a town meeting was called to discuss opening surrender negotiations. The Corinthians arrived in time to stop them. The Spartan general, Gylippus, landed up the coast and raised a force of some 2,000 to fight with him against the Athenians. He marched onto the high plateau just to the west of Syracuse, so surprising Nicias that the Athenians were unable to stop them from reaching the city. Gylippus oversaw the construction of a counterwall to stop the Athenian effort, which split the plain and gave Syracuse control of the northern half and access to the rest of the island.

In the summer of 414 Nicias counseled Athens to abandon the invasion or to dispatch a second force equal to the first. Athens sent another force, but Sparta broke the truce and attacked to delay or cancel it. In Sicily the summer was spent wall-building and Gylippus got the better of it. In the spring of 413 he was ready to attack the Athenians. This he did with a two-pronged assault, starting with his navy attacking Nicias' base at Plemmyrium on the southern end of Syracuse's harbor. When the defenders left their positions in order to watch the sea battle, Gylippus attacked and captured the town. While the Athenian fleet badly hurt the attacking Corinthians, they had lost their base. They were forced to move to another mooring deeper in the harbor, which severely limited the navy's superior maneuverability. This was the beginning of the end for the Athenian military effort in Sicily.

Hearing of the dispatch of the second Athenian force, Gylippus was determined to win before they arrived. He launched another attack on the Athenians, this time using his army first to distract the Athenian sailors. Again he was successful, for when his ships entered the harbor bent on close-quarters action with the Athenians they found a disorganized navy scrambling to man its ships and engage. The engagement lasted intermittently for three days, with the combined fleet of Corinth and Syracuse gaining the upper hand. At close quarters they were able to rain arrows down on the Athenian ships, killing large numbers of sailors. It was a major but not decisive defeat. When the second force under Demosthenes arrived in July 413, Athenian hopes revived.

Demosthenes took command of the entire Athenian force from an ailing Nicias and immediately went on the offensive to regain control of the plateau overlooking the city. He sent a force secretly up the steep slope of the western end of the plateau that captured a surprised Syracusan force at the town of Euryalus. However, Gylippus ordered a counterattack and in the confusion of the night fighting the Athenians lost their momentum and the battle. Unable to reestablish a position on the plain, Demosthenes loaded his army onto ships to return home, but stopped at the last minute because of a full lunar eclipse on 27 August 413. The superstitious Nicias decided to wait a month before sailing, giving Gylippus his last chance to win. He anchored his fleet across the mouth of the harbor and chained the ships together. Again the small harbor kept the Athenians from using their superior maneuverability; they lost fifty ships to the Syracusans' twenty-six. At that point the Athenian crews refused to fight and demanded a retreat by land. On open ground Gylippus separated the Athenians into two groups and defeated each individually.

Of the 45,000 to 50,000 men Athens had sent to capture Syracuse and Sicily, only 7,000 survived the final battles. They were all sold into slavery except Nicias and Demosthenes, who were killed against Gylippus' orders. The Syracusans, poorly prepared for the war and on the verge of surrender, survived thanks to the Spartan Gylippus. The city remained free and a major power in the western Mediterranean for another two centuries, until crushed in the Punic Wars of the third century b.c.

Athens' fortunes waned. The ships and crews lost in Sicily were not irreplaceable, but Athens found her naval power and prestige badly damaged. Delian League members seized the opportunity to break away, Persian forces opened an offensive to regain their lost territory in Asia Minor, and Sparta began investing in sea power, with both their own ships and others provided by Persia. That refocused Athens' attention on her eastern possessions, and when her lost ships were replaced that was where she sent them. The replacement crews, however, did not match the quality of those lost in Sicily, and Athens was destroyed as a naval power at Aegospotami in 405. There, in the Hellespont connecting the Aegean and the Black seas, a Spartan fleet commanded by Lysander attacked the last Athenian fleet, capturing intact 170 ships while their crews were ashore. Without their fleet, the city of Athens could not resist the siege that Sparta was then conducting. The city held out through the winter of 405-404, but surrendered in the spring.

Had the Athenians followed Lymachus' advice and attacked as soon as they landed, Syracuse certainly would have fallen and a major Athenian power center would have been established. Carthage and Rome were at the time mere shadows of their future selves; it would have been little problem for Athenian ships and troops to control them. With the supplies and manpower available from these acquisitions, no one would have been able to match Athenian power, and Alcibiades' empire could have come to pass. Rome may never have built their empire and Europe, at least, would never have been the same. Instead, the defeat ended Athens' reign as the dominant polis. No one successfully dominated Greece until Philip of Macedon arrived in the 340s to unite the polises under one ruler; his son Alexander built an Eastern empire instead of the Western one Athens would have built. Thus, the fate of Asia changed as well.