A good portion of blame for Athens losing the Peloponnesian Wars can be laid at the feet of two men: Alcibiades and the Spartan king, Lysander. Alcibiades was the nephew of Pericles. He was very charismatic and the young people of Athens loved him. But he had a glaring flaw; he lost his parents at a young age and became something of a sociopath, not caring what his deeds resulted in. As stated before, he enjoyed the backing of the young of Athens, but this had a polarizing effect. The older generation did not approve of Alcibiades. He would have raucous parties that furthered the rift between Alcibiades and the older generation of Athens.
At one of these parties Alcibiades and his cronies concocted an ill-advised plan. They would sail to Syracuse, in Sicily, which was over 500 miles away, with a huge army of over 30,000 men. Syracuse was supposedly easy pickings and an ally of Sparta. The main opponent to this plan was a man named Nicias. When Nicias voiced his concerns over the invasion plan, Alcibiades turned the tables against him and Nicias ended up becoming a co-commander of the invasion.
The night before the fleet set sail, there was a rash of vandalism in Athens. Statues of Hermes were destroyed and defaced. This was seen as a horrible sign because Hermes was the god of travel. However, the fleet left anyway. The aged assembly soon blamed Alcibiades for the attacks and recalled him from the attack fleet. Alcibiades convinced the retrieval group to allow him to take his own ship back to Athens. As they neared Athens, he kept sailing and ended up in Sparta. He quickly rose in prominence and soon had the king’s ear.
Meanwhile, the Athenians were making strides against Syracuse. But Alcibiades gave the Spartans all of the information on the invasion force, and they sent a commander to help Syracuse. He restructured the army using Spartan strategies. This along with full knowledge of the enemy helped to crush the Athenian army. Most of the army was destroyed at Syracuse, but some of them managed to escape. They tried to retreat to Athens overland, but were chased down and destroyed. The Syracuse debacle was a huge setback to Athens in terms of resources and manpower. Slave rebellions started to become commonplace. Without ready working hands, silver mine production started to peter out. General unhappiness reigned and the oligarchic element began to take hold in the Athenian political realm, even resulting in a failed coup.
The second nail in the Athenian coffin was the Spartan, Lysander. The very Spartan paradigm was rigid; they were known for taking a slender view of things, one battle at a time. Lysander had a great strategic mind and thought much more broadly than his contemporaries in Sparta. He saw that the strength of Athens was their navy; however reduced it was by the devastation of Syracuse. Since Sparta was relatively poor in liquid wealth, Lysander found a friend in the ambitious Persian satrap Cyrus, and forged an alliance with Persia which helped him build the Spartan fleet.
The Spartan fleet was eventually able to either seize or destroy the Athenian fleet. The Spartan navy then proceeded to blockade Athens, cutting it off from most supplies, most notably grain. This combined with civil strife led the Athenians to finally surrender. Instead of destroying the city in Greek tradition, Lysander spared most of the population due to political pressure from within Sparta and the fear of a power vacuum that would form in the absence of the other Greek power.
The immediate result of the surrender of Athens was clear. They were to pull down their long walls, which eliminated most of their land-based defenses. They also had to allow all exiles back to Athens, many of whom were pro-oligarchy. The war also lead to a shift in naval power; where Athens ruled the seas supreme since ancient times, the Spartans now had a powerful navy. It can be argued that there were no real winners in the Peloponnesian Wars. The Spartans set up an oligarchy in Athens ruled by the Thirty. This set democracy back in Greece somewhat. It also set most of Greece back in terms of population and financially for a good time.
A brief epilogue to Alcibiades. He had to run from Sparta upon being suspected of impregnating the Spartan queen. He then became affiliated with the Persian satraps, but fell out of favor. He was welcomed back to Athens as a savior, but the climate quickly soured for him when he was blamed for heavy naval losses. He went into exile and was later assassinated.