In the year 425 BC the pattern of the war began to change. With resources now stretched on both sides the cruelty and savagery of the war began to escalate. Both sides began punishing their opponents with increasingly barbarity - enslaving women and children, even murdering the entire male population of some cities.

Sparta suffered a major setback when the Athenians surrounded the island of Sphacteria and forced 420 elite Spartan hoplites to do the unthinkable and surrender. But this advantage was soon lost as a new generation of younger Spartan generals got their first taste of war. They began encouraging members of the Delian League to leave the alliance, promising that Sparta would allow them to run their own internal affairs, even if they were democrats.

Only a year later this new tactic had already caused several city-states and colonies to abandon the Delian League, forcing Athens to agree to an armistice in 423.