How did the Athenians stop the Spartans from beating them when the Spartans army attacked in the first year of the war

The subsequent failure of Sparta to reduce Athens in the first decade of the war led to a brief peace (421 B. C.). But soon the Athenians intervened in the Peloponnesus (418 B. C.) and attacked neutral states in the Aegean ... In the detritus of the Peloponnesian War, the agrarian fighting of the old polis was ended.