Although their workforce had been depleted by the plague, and there were far more pressing problems closer to home, the Athenians were tricked into a city Sicilian state to invade one of their neighbors, so with the promise of gold and glory, the Athenians set sail and lingered along the coast of Sicily. Due to unstable leadership the Athenians were unable to land when they had the advantage, but still achieved some early victories, encouraging both the leadership and the hoplites, everything changed at Epipole. Like any other battle, the Athenians opened up well, but were lured into pursuit to defeat their enemies (my guess is a false rout, underhanded tactics) by which point the hoplite advantage was gone, the Athenian army was forced to defeat, suffering major casualties. The routed Athenians were again defeated at sea and the remainder managed to limp back to Athens. The problem with the realist perspective on the Peloponnesian War is that the chain of events that led to Athens' defeat and their demise as a vital power in the Hellenic world was beyond any kind of human control; fate is a factor that a realist would overlook,