In 421, the Archidamian War, which had lasted ten years, came to an end. Sparta had tried to defeat Athens and liberate the Greek towns belonging to the Athenian empire, but had miserably failed. In the six years following the Peace of Nicias, the Athenians tried diplomatic means to force back Spartan power, but they failed too. In 413, war was renewed: Decelean or Ionian War, that ended with the fall of Athens.
Although the Peace of Nicias had been signed, it comes as no surprise that the years after 421 witnessed a continuation of the war with diplomatic means. Athens and Sparta had sworn to cooperate and had joined a defensive alliance, but on both sides, there were politicians who wanted to resume the war. The Spartans did not give back Amphipolis, as they had promised, so the Athenians felt free to continue the occupation of Sphacteria and Pylos.
Moreover, Alcibiades, a nephew of Pericles, Olympic winner, and rival of Nicias, launched a new policy that promised the collapse of the Peloponnesian League without much direct Athenian involvement. This was possible because diplomats from the Peloponnesian city Argos, Sparta's eternal enemy, had concluded a treaty with Mantinea, which had reasons to fear a Spartan attack. Both towns shared a democratic constitution, and Alcibiades saw them as natural allies. Not much later, Elis, another democracy, joined the Argive alliance, and Athens became the fourth member of this coalition.
Athens had found friends in the Spartan backyard and cut off the route between Sparta and its northern allies Corinth and Thebes. It was not likely that the Spartans would acquiesce. In fact, it would be easy for them to make the coalition fall to pieces: if they attacked one of its Peloponnesian members, the Athenians were forced to choose between either their Spartan alliance (which meant abandoning its allies), or supporting Argos, Mantinea, and Elis, and risking an open war with Sparta in the Peloponnese. As it turned out, Athens, frustrated by the Spartan unwillingness to carry out the terms of the Peace of Nicias, preferred the second alternative, and when the young Spartan king Agis II marched to the north, the Athenians supported their fellow-democrats. In 418, a battle was fought at Mantinea, and Agis decisively defeated his enemies (text).
As a result, Sparta restored its prestige, the quadruple alliance was dissolved, and democracy suffered a severe blow. The prestige of Athens needed a boost, and in 416 it attacked the little isle of Melos, which had until then remained outside the Delian League, and had - in fact - at times offered financial support to Sparta. Athens could not allow its authority being flouted and subdued the island, eventually killing all male inhabitants.
Meanwhile, the Athenians had made the crucial mistake for which they would in the end pay with the loss of their empire. In 420, the satrap of Lydia, a man named Pissuthnes, had revolted against the Achaemenid king Darius II Nothus (423-404). The great king's representative Tissaphernes had arrested the rebel and sent him to Darius, who had him executed. Pissuthnes' son Amorges, however, continued the struggle and received help from Athens. This was a break with the traditional non-aggression between the Athenians and the Persian king, and its main result was that Darius ultimately sided with Sparta.