The Athenians used the captured Spartiates and their helots as hostages from their capture in 425 to the Peace of Nicias in 421. The Athenians repeatedly threatened to kill the Spartiates hostages, whom they had taken to Athens, if a foreign army marched into Attic soil, which effectively put an end to the Peloponnesian invasions of the Attic mainland, which had been sharply dividing the population of Attica politically since the beginning of the war. Furthermore, after the defeat of the Spartans at Pylos the entire promontory and the island of Sphacteria were seized as an Athenian naval base, at which were stationed a force of marines and Messenians. Demosthenes' decision to occupy a permanently fortified position behind enemy lines allowed the Athenians to weaken Spartan support by providing helots and disgruntled allies with a place to retreat to, as well as allowing the Athenians to raid any inland point on the Peloponnese at will (they were already capable of raiding the entire coastline and had been doing so for several years). As a result the Athenians seized and fortified several enemy-held fortresses within the next few years, threatening to kill the Spartiates if there was an attempt to take the positions. In 421, after both sides were severely exhausted, the Peace of Nicias dictated the return of the Spartiates in return for the Spartan evacuation and subsequent return to Athens of her cleruchy at Amphipolis. Unfortunately for the Athenians, the Spartans broke the treaty, since after the Athenians had handed over their hostages the Spartans refused to hand over Amphipolis, changing the treaty at the last minute.