Changes

The tensions between the Athenians and the Greek only grew. Thucydides noted that many believed that war was only a matter of time and that the Thirty Years Peace Treaty would soon be broken. There was a crisis when a Greek colony in Thrace that was in dispute with Athens asked the Spartans for assistance. Athens despite this laid siege to the colony. The Athenians, at this time were also in dispute with the small city-state of Megara and they unilaterally banned the ships of that city from its port and its allies. This became known as the Megarian Decree. Megara was a long-time Spartan ally and this was widely resented, as it was seen as an attempt to make Megara completely dependent on Athens.<ref>Thucydides, The Peloponnesian War 1.71-73 </ref> This was not acceptable to Sparta and they believed that if Megara came within the orbit of the Athenians that they would use the port to weaken their position in Greece.

The Athenians were extremely confident and they knew that as long as they had their navy and their ‘Long Walls’ that they could not be defeated even if they could not beat Sparta and her allies on the battlefield. This strategy was recommended by Pericles to the Athenians and was much praised by Thucydides. Sparta began to contemplate war but they seemed unwilling to formally declare war. Then the situation spun out of control when allies of Sparta attacked the allies of Athens. The Spartans came to believe that they had no choice but to go to war. In 431 BCE, the senior Spartan king led an army into the countryside around Athens and laid it waste. This was the start of the great Peloponnesian War. The early years of the war were a stalemate because according to Thucydides writings, this was because the Athenians followed Pericles cautious strategy.<ref>Thucydides, The Peloponnesian War 2.69–71</ref> Later Athens, encouraged by Alcibiades launched the Sicilian expedition to conquer Sicily during a lull in the fighting, known as the Peace of Nicias. This proved to be a disaster and it led to the loss of an Athenian army and navy. Remarkably, the Athenians continued to fight and the Spartans needed Persian help to defeat them.<ref>Thucydides, The Peloponnesian War 6.6–11</ref> The destruction of the Athenian fleet at Aegospotami ended the war, and Athens surrendered the following year. Athens was forced to tear down its Long Walls and was fortunate not to be utterly destroyed.