Final answer:
The Spartans made a deal with the Persian Empire to secure financial support for a larger fleet, leading to a decisive victory against Athens and the end of the Peloponnesian War. However, Sparta's failure to return the Ionian cities to Persian control led to Persian support for Greek resistance, eventually compelling Sparta to uphold its promise to Persia.
Step-by-step explanation:
The critical turning point came after the Athenians faced a devastating loss in Syracuse. The Spartan leadership, realizing the need for a larger fleet to counter Athens, turned to Persia for financial support in exchange for Ionian cities' autonomy.
In 405 BCE, backed by Persia, the Spartans crushed the Athenian navy at the Battle of Aegospotami, leading to Athens' surrender and the end of the Peloponnesian War in 404 BCE.
However, relations between Sparta and Persia soured when Sparta failed to cede control of the Ionian cities back to Persia as promised. This breach of agreement led Persia to fund Greek resistance against Sparta, forcing them into a position where they had to honor their initial promise to secure continued Persian support.