Aigospotamoi: the final battle of the Peloponnesian War (431-404). In September 405, the Athenians were decisively defeated by the Spartans and lost their navy. As a result, the siege and fall of Athens became inevitable.
The Road to Aigospotamoi: Athens against Sparta
Since the beginning of the Peloponnesian War in 431 BCE, the Greek world had endured much. The intensity of the war was unprecedented. Earlier wars had consisted of little more than skirmishes, hardly ever resulting in a large-scale battle. During the Peloponnesian War, however, battles were constantly being fought throughout Greece. The adversaries were two fairly evenly matched alliances: the Delian League and the Peloponnesian League. Both had recruited large armies and built large navies. The war had ebbed back and forth. Victory and defeat had alternated in rapid succession. At first the alliance led by Athens had appeared to head for victory, but a number of setbacks had ruined all hope of triumph. In particular the failure of the Sicilian Expedition in the years 415-413 BCE brought the odds back in Sparta's favor; worse, the Amorges affair had brought the Achaemenid king Darius II Nothus into the war as an ally of Sparta.
To some extent, the stalemate between Athens and Sparta had been preserved because each persevered in a different strategy. Athens was a maritime power; commerce and overseas territories were essential to the upholding of Athenian wealth. So a strong fleet had been built to control the seas. Sparta's dominance, on the other hand, was based on her well-drilled land forces; the supremacy of the Spartan hoplite was undisputed.
The alliances were so evenly matched that a definite outcome was only possible if one side broke the other's sphere of influence. In other words, Athens would need to defeat Sparta on land, and Sparta would need to deprive Athens of her control of the seas. But the misfortunes that had struck Athens burdened her in such a manner, in terms of manpower and financial means, that she was forced into a defensive position; defensive in the sense, at least, that Athens could not force a decision on land.